Thanos is here and it's his fault.
Loki has known guilt before; guilt so heavy he couldn't breathe; guilt so heavy that he was sure he had to die before he could ever be free of it. Before he could ever smile again without choking on it.
But nothing, nothing he has ever done, comes even close to seeing the last of Asgards people, laying there, dead at his feet and around Thanos; mouths open in screams of terror. He has never done anything has unforgivable as this. And all because he thought himself so intelligent, so clever, that he could bargain with the mad Titan himself. All because he thought they would need something, when Thanos arrives; something that he could hold over the Titans head, something that would force Thanos to let everyone live -
But Loki hadn't known how close Thanos already was. He hadn't known that the Tesseract would lead Thanos right to them; that he would massacre everyone, every woman, every child, in but a blink of an eye; before Loki could step forward and try to do something ... Loki hadn't known that every last Asgardian would be dead before Thanos would start asking questions.
Loki knows that giving Thanos the Space Stone was the stupidest he has ever done; worse even than letting himself fall into the abyss. The Titan has two Infinity stones now and Loki knows in his heart that he will get the rest of them, too, when he leaves their wreck of a ship. The Reality Stone will be next and he will have it in a matter of minutes; the Collector will not be able to protect it any longer. Two stones are on earth and while the Avengers are strong, and while they might even defeat Thanos children, as soon as the mad Titian himself comes, Earth will fall. The location of the Soul Stone is unknown, but Loki has no doubt that Thanos has plans for that; if he doesn't already know where to find it.
The space stone inside the Tesseract was probably the best protected before Loki took it in all his vain stupidity; even after Asgards fall it would have been safer lost in the depths of space than here where he had given it up just so he wouldn't have to watch Thor get hurt.
Loki knows that he is safe right now. Thanos has all but forgotten about him now that he has fulfilled his purpose. He could stay hidden and wait till the Titans leaves before escaping with Thor. He knows that there is a ship on it's way that will help them. He could wait. He should just wait. When they are safe, they can plan, they can fight together, he and Thor -
“You are just the worst brother.”
Never has anything Thor said rung truer than that. He hadn't meant to bring Thanos here; hadn't meant for everyone to die - but what does that matter when the outcome is all the same? Thor hurt, lost, Heimdall dead, all of his people dead -
He can see Thor from his hiding place. He is trapped in a metal cage; gagged, unable to do anything at all. Good.
Loki knows that he will die. What he plans (if anyone would be so generous as to call his desperation a plan) is stupid, reckless and, quite frankly, suicide. But if Thanos leaves here it will be a matter of hours before he has all six stones in his possession; a matter of hours before all he has ever known is lost. He will die either way.
Loki steps forward.
The moment Thanos eyes focus on him, all thought leaves him. He knows that he is saying something, that he pledges his loyalty to him, but it hardly matters, as long as he gets close enough - He can see Thor from the corner of his eye. It's the first time since Thanos tortured him that Loki is able to see his face. And it hurts.
He never thought he would see his big brother so defeated, so desperate, so lost. He is close to tears, Loki knows, and he hasn't seen Thor cry since they were both children and Loki had gotten himself hurt on a journey with just the two of them -
He is staring at Loki and Loki knows that if he has to die, he cannot do so without saying something to his big brother.
“I am Loki. Prince of Asgard. Rightful king of Jötenheim. God Of Mischief”, he breathes and turns his head fully to look at Thor, “Odinson.” He lets Thor see his dagger as he conjures it and he can feel Thors desperation grow, can feel Thor will him to stop; stop this madness, turn around, stay safe- but he can't. He can't let Thanos leave here, just to collect the other four stones and doom the universe. He cannot. Not when it is his fault that he is here in the first place, not when it is his fault that all he can see is bodies - the bodies of Thors people. He cannot.
He just hopes that Thor will look back at this moment and hear all the things Loki can't say right now. He hopes that Thor will understand that he claimed the name Odinson not for Odin, not for himself, but for Thor. Thor is the last Odinson and Loki wants to be his kin, wants to be his brother in his final moments. He hopes Thor will see that. He hopes Thor can hear all the things that go with it.
“I am your brother; I never stopped being your brother.”
“I am sorry.”
And, most importantly,
“I love you.”
He hopes that Thor will look back and see and hear all that. Hopes that Thor will know that Loki never meant for this to happen and that Loki is glad that he had Thor as his brother, even if Thor will not feel the same.
He is not surprised when Thanos catches his arm before the dagger makes contact with the Titans throat. He knew it was impossible to draw so much as a single droop of blood from a Titan all alone with nothing but words and a dagger. But he had to try for all the people that died today because of him, had to try for all the people who's death was jet to come. He had to try for Thor.
He doesn't regret it; not even when Thanos hand closes around his throat and his feet are lifted from the ground. He is sad that Thanos face will be the last thing he will ever see; his nightmare come true. But he cannot regret it.
“I am sorry. Goodbye.”