Suicide
Loki had always had suicidal tendencies, although he hadn't often acted on them. Not consciously, at least. Maybe he was more reckless than usual, maybe he provoked someone that he shouldn't have, maybe he tried an especially risky spell. Unbeknownst to him, there had been an ingredient of self.loathing and self destruction in all of those actions.
After he discovered his true origin, it got much worse. He had always felt different, wrong somehow, but at least back then he'd been an Asgardian royal. When he found out that he was actually a Jotun cast out, his self loathing sky rocketed, to levels that were very difficult to control.
And then he threw himself into a void, hoping that death would solve everything. In death he wouldn't have to hate himself, in death he wouldn't have to curse himself for his wrong decisions, all the relationship he had destroyed, all the worlds that never liked him or cared about him like they did his whole family. In death, he would finally be free from himself.
But he didn't die, and everything got worse. He constantly needed distractions, missions,people and worlds and conquest that would quiet the voices telling him that he should try again, that he was a monster that deserved only agony and death. And so he did, and distracted himself nicely in Midgard, trying his best to forget what happened before and after he fell. Everyone said he was crazy, no one looked into the why or the how.
The cell was a bad moment for him. Frigga's death was even worse, telling him that maybe she wouldn't have died if he had properly killed himself before. Guilt threatened to eat him whole when Thor came, and offered another distraction and his “death” in Svartalfheim offered a little moment of closure. By almost dying, he felt he was atoning, even if only a little for his mother, and somehow fixing things with Thor.
Then the good times came. When his people aplauded him, and they were sad when they saw his death on that play. When he felt free, and unburdened from himself, because he was actually being someone else. It helped him build up confidence, and recover from darker times. When Thor and they inevitable got involved on an adventure, he wanted to stay alive until the end of it.
And then he went back to Midgard, and things became... worse. He had to deal with the threat of Thanos (he will make you long for something as sweet as pain), the hatred of the people of Midgard, the mistrust of his brother's allies. He had to be smart, like he always was, but prat of him didn't want to disappoint Thor after making up with him, and their enemies were so powerful, and there were so many of those heroes who wanted him dead....
He fought, and he fought on Thor's side, and gained some smiles, some pats on the back. But no one really liked him, and no one really trusted him, and he was tired, and in a place he didn't belong, and things started going downhill again. The voices were back, telling him that maybe it would be better to leave things here, before he disappointed Thor again, before some of those anti-Loki grous got the better of Stark's technology.
In death, no one would have to fight over if he was supposed to be given asylum or jailtime. He wouldn't have to constantly prove himself to all of the worlds, he wouldn't have to hear insults, he wouldn't have to mistrust every compliment. No one would remind him ofthe horrors he'd done in the past, nobody would be wary of him again, or make a disgusted face. He would be fixing problems, helping people.
And maybe, just maybe he could finally be at peace.
Thor was frozen in place when they found Loki nearly bled out in his room in Avengers HQ.
Bruce and Natasha were quick to act, trying to stop the bleeding, seeing how much blood he'd lost, seeing if he was responsive. He wasn't. And there was so much blood everywhere, and Loki was so pale, but it ws all off, because he seemed... almost happy. Like an angel sleeping after a vey long day. Neither of them thought too much about the situation as they took him to the medbay Tony had instructed.
Tony was busy calling the doctors in the world, in all of the worlds, and telling them that his was a life or death suation and to hurry the hell up. He was feeling horribly guilty – he had seen how sad Loki was lately, and had only offered alcohol and a friendly ear. He should have done something more to stop those anti Loki protests, made more pro-Loki propaganda, given him more validation. Something that would have prevented this. Now they could lose an incredible ally, and someone who had become a friend. It was bullshit.
“Thor?”
Steve appeared behind the god of thunder, trying to get him to react. But he understood Thor. Just looking at Loki's bloodied bedroom was difficult and painful for Steve, and he hadn't grown up with the guy. But he liked Loki, and how complex and dramatic and impossible he was, and it was disheartening losing him to... this. Something that shouldn't have hurt him so much, something that could have had a solution. Steve knew many things about dealing with trauma after Bucky, if he had known... he would have helped Loki, gladly. Now maybe it was too late.
Bruce understood the feeling. He'd been there too. He only hoped Loki would make it, and that things would get better with time. He didn't wish that hell on anyone. Self loathing could be worse than a million alien robots, monsters and gods. More destructive.
Wanda and Peter Parker had been informed, too, and aske what they could do, but the answer was nothing. They held a vigil anyways, told the world that Loki, one of their heroes in the fight against Thanos was now fighting for his own life, and in a critical situation. There were flowers and prayers for him that night. Wanda and Peter hoped that wherever he was, and whatever happened, this would bring him some comfort. He needed to know that they cared, very much so, and that they really didn't want him to go.
Those amazing doctors Stark had summoned told Thor that they had done all they could, that his brother was now in a comatose state and that they didn't know when or if he would wake up again. Thor looked at the ghost of his brother in that bed, pale, quiet and it broke his heart in one million different ways. He knew that Loki had a tendency to melancholy but he hadn't thought....
He took his brother's cold hand on his, planted a soft kiss, willing Loki to live. He had so much to do yet, so many people to astonish, so many featsof bravery and wonder. And they were supposed to do them together, side by side, as brothers. He hadn't been there for him, hadn't seen the signs. Thor had been a fool and he could be losing themost precious thing he had. And there so many things left unsaid, so many promises, so many thanks he hadn't given. Thor tried not to cry, as he sat by his borther's bedside, listening to the beeping of the machines.
It hurt to think that Loki hated himself enough to do something like this. That he had been in so much pain, so much sorrow. And that they hadn't been there to see it.
Back in Loki's old room, there was bloodied note, handwritten by the god of mischief: This will fix everything.
But instead, it had broken everything.