Riddle was abused and/or bullied during his time at the orphanage.
In the books, there is no evidence of either. Discussion with Mrs Cole paints him as the bully, not the victim. Riddle's own words confirm this:
I can make bad things happen to people who annoy me. I can make them hurt if I want to.
HBP, chapter 13: The Secret Riddle
These aren't the words one might expect to hear from a scared, bullied child. Even at eleven, Riddle was obsessed with power:
Riddle stared from the wardrobe to Dumbledore, then, his expression greedy, he pointed at the wand.
HBP, chapter 13: The Secret Riddle
Dumbledore burned Riddle's belongings during his visit to the orphanage.
When Dumbledore said that he was a wizard during their first meeting, Riddle demanded proof. Dumbledore set Riddle's wardrobe aflame, but when he cancelled the spell, the wardrobe turned out to be intact:
the flames vanished, leaving the wardrobe completely undamaged.
HBP, chapter 13: The Secret Riddle
Dumbledore presumably used a charm to conjure cold fire or an illusion of fire. The only thing that could have been hurt by it was Riddle's pride.
Riddle turned evil because Dumbledore set his wardrobe on fire.
By the time Dumbledore handed him the Hogwarts letter, Riddle had already killed a rabbit and traumatised two children so badly they refused to speak about the incident.
Dumbledore's idea certainly didn't endear him to Riddle, but it can hardly be considered enough of a reason to set the boy on a dark path.
Riddle first opened the Chamber of Secrets in his sixth year.
In the books, it is stated multiple times that Riddle was sixteen when he turned the diary into a Horcrux; he even says so himself:
I decided to leave behind a diary, preserving my sixteen-year-old self in its pages
CoS, chapter 17: The Heir of Slytherin
Harry's birthday is in July; he's eleven for the entire first year at Hogwarts, and turns twelve before the next school year. It feels natural to think that a sixteen-year-old Riddle would be in his sixth year, but that's not necessarily true. He was born on the 31st of December 1926. In the UK, children born from September to December go to school a year later; this is also the case with Hogwarts. As such, Riddle would be fifteen when he started his fifth year and turn sixteen after the first term.
In (CoS 13), Riddle – described as "about sixteen" – asks Headmaster Dippet to stay at Hogwarts during the summer holidays. Dippet declines because he is concerned about Tom's safety; it can be inferred that the series of basilisk attacks happened recently. This means the memory is likely from June, before the end of the school year.
If the attacks stopped before the summer holidays and Riddle was sixteen at that time, he would be in his fifth year.
He confirms this:
It had taken me five whole years to find out everything I could about the Chamber of Secrets and discover the secret entrance...
CoS, chapter 17: The Heir of Slytherin
This results in the following timeline:
31st December, 1926 – Riddle is born
1st September, 1938 – beginning of Riddle's first year (11 years old)
1st September, 1939 – beginning of Riddle's second year (12 years old)
1st September, 1940 – beginning of Riddle's third year (13 years old)
1st September, 1941 – beginning of Riddle's fourth year (14 years old)
1st September, 1942 – beginning of Riddle's fifth year (15 years old)
~ June, 1943 – Riddle opens the Chamber of Secrets (16 years old)
Dumbledore told Riddle that he couldn't stay at school during the summer.
Riddle had that conversation with Dippet, not with Dumbledore (CoS, chapter 13). That would not be Dumbledore's decision to make, as he wasn't the headmaster at that time.
Riddle turned evil because Dumbledore refused to give him the DADA position.
As of his second job interview, Voldemort had created at least three Horcruxes: Diary, Ring, and Diadem. He had also began gathering followers in order to overthrow the Ministry of Magic.
Voldemort had a human or almost-human appearance during the First War.
One of the memories Dumbledore shows Harry in book six is that of Voldemort requesting the position of the DADA professor. The scene includes the description of Voldemort at the last stage of his physical transformation prior to turning into the monster seen during the events in the books:
Voldemort had entered the room. His features were not those Harry had seen emerge from the great stone cauldron almost two years before; they were not as snakelike, the eyes were not yet scarlet, the face not yet masklike, and yet he was no longer handsome Tom Riddle. It was as though his features had been burned and blurred; they were waxy and oddly distorted, and the whites of the eyes now had a permanently bloody look, though the pupils were not yet the slits that Harry knew they would become. He was wearing a long black cloak and his face was as pale as the snow glistening on his shoulders.
HBP, chapter 20: Lord Voldemort's Request
As this takes place before the First War, this is the most human appearance he might have during his rise to power – short of using the Polyjuice Potion or other methods of disguise.
Several passages point to Voldemort looking fully serpentine during the First War.
The first piece of evidence is the reaction of the Death Eaters summoned to the graveyard in Little Hangleton: while they are hesitant to approach Voldemort at first, they appear to recognise him immediately (GoF, chapter 33). It could be argued that it wasn't difficult to guess who he is because he used the Dark Mark to call upon them; however, in the same chapter, Voldemort says:
I would settle for my old body back again, and my old strength.
GoF, chapter 33: The Death Eaters
As there is no indication that the ritual failed, aside from unintentionally tying Harry's life to Voldemort's, it can be presumed that Voldemort's snakelike body was the desired and expected result. Notably, he calls it "(his) old body", which suggests that he had the same appearance prior to being disembodied by the rebounding curse.
In the seventh book, Harry dreams about Halloween 1981 from Voldemort's perspective; the dream begins with Voldemort on his way to murder the Potters. He encounters two muggle children trick-or-treating; one of them compliments him on a nice costume, then gets close enough to see his face and subsequently flees in terror.
He saw the small boy’s smile falter as he ran near enough to see beneath the hood of the cloak, saw the fear cloud his painted face: then the child turned and ran away …
DH, chapter 17: Bathilda's Secret
Would Voldemort's earlier waxy and blurred appearance – seen at night, obscured by the hood, so in poor lighting – be enough to frighten the child so badly?
Moreover, at the end of the fifth book, an auror yells that he saw Voldemort in the Atrium. Fudge replies:
‘I know, Williamson, I know, I saw him too!’
OotP, chapter 36: The Only One He Ever Feared
If, of all people, Fudge was able to recognise Voldemort at a glance, it should be safe to assume that Fudge knew of Voldemort's serpentine appearance – either from the First War, or from the Death Eater trials that followed it.
Voldemort punished his followers severely for trivial reasons.
Peter Pettigrew talks back to him without consequences:
‘I found you,’ said Wormtail, and there was definitely a sulky edge to his voice now. ‘I was the one who found you. I brought you Bertha Jorkins.’
‘That is true,’ said the second man, sounding amused.
GoF, chapter 1: The Riddle House
Voldemort doesn't appear angry; he knows he is the one in power, despite being stuck in a barely usable body at the time.
In the first chapter of Deathly Hallows, Snape and Yaxley are almost late to the meeting; Voldemort merely comments on it and moves on. Later in the same chapter, it turns out Pettigrew couldn't manage a Silencing Charm on Ollivander - for some reason - and all Voldemort does is send him away to fix it.
See this thread for more information.
Voldemort was gone for 14 (full) years.
This error arises from approximation. As explained in this thread:
He was killed on Halloween 1981, and was revived on the 24th of June 1995, so he was gone for 13 years and 8 months, which people round up to 14 years (1981-1995).
Voldemort had a human or almost-human appearance before his resurrection in June 1995.
When Quirrell removes his turban during the confrontation in the third floor corridor in 1992, the parasite Harry sees is hardly human in appearance. Voldemort’s face is described as
chalk white with glaring red eyes and slits for nostrils, like a snake.
PS, chapter 17: The Man with Two Faces
This passage matches his post-resurrection appearance in later books.
Horcruxes made Voldemort insane.
There is nothing in canon that indicates this. A more likely explanation is that Voldemort's behavior changed because he no longer needed to hide what he was.
See this thread for an in-depth analysis.
Works cited
PS: Rowling, J. K. Harry Potter and the Philosopher's Stone. Pottermore Ltd, 2012
CoS: Rowling, J. K. Harry Potter and the Chamber of Secrets. Pottermore Ltd, 2012
GoF: Rowling, J. K. Harry Potter and the Goblet of Fire. Pottermore Ltd, 2012
OotP: Rowling, J. K. Harry Potter and the Order of the Phoenix. Pottermore Ltd, 2012
HBP: Rowling, J. K. Harry Potter and the Half-Blood Prince. Pottermore Ltd, 2012
DH: Rowling, J. K. Harry Potter and the Deathly Hallows. Pottermore Ltd, 2012