Avengers: Infinity War Trailer - 11 Major Plot Reveals
9. Loki Gives Thanos The Tesseract
Oh Loki. Just when you thought he was really on the side of good, there he goes handing over the Tesseract to his ol' buddy Thanos.
The first, and easiest, reading of this is that the God of Mischief is up to his old tricks and has gone back to the side of evil, siding with the Mad Titan and helping him in his quest to collect all the Stones - and really, it wouldn't be beyond the character to do that.
But there's also the chance, and it's more interesting from a character point-of-view, that he still wants to do the right thing but, seeing all the carnage around him (notice multiple bodies in the same scene) he's been left with no other choice than handing over the Tesseract after failing to stop Thanos at this point. In fact, this could even be his final stand; a futile attempt at using the Tesseract against Thanos.
And if Loki were to ever die, this could be a powerful moment to do it.