Things look grim for Loki and Thor during the postcredits scene of Thor: Raganarok. As they're optimistically flying out the Asgardian survivors to Earth, they bump into a spaceship manned by none other than Thanos. He's come to get the stone that Loki takes from the Tesseract from Odin's vault in Thor: Raganarok.
There's a theory out there that Thanos takes his revenge out on Loki, who had formerly allied with him, and makes Thor watch. In the first Infinity War trailer, we see Loki handing over the Tesseract to a character, likely Thanos. While some fans take this to be a sign of him betraying his brother, others see this as him reluctantly giving up the stone. This makes sense as well, given his recent trajectory towards goodness.
It's possible that the trickster god is ultimately trying to save his brother from Thanos, who we see holding Thor's head in a pretty uncomfortable position in the second trailer. This may be Thanos forcing Thor to see something he doesn't want to see, rather than him, you know, trying to crush his head or snap his neck. In this same theory, Loki exchanges the stone for his brother's life, and while sparing Thor, Thanos will unleash his wrath onto Loki. If Loki has indeed betrayed Thor in Infinity War, this scenario would provide him with a redemptive arc.
