Poor Loki. It has recently been confirmed that the God of Mischief was mind-controlled to take the Tesseract in the first Avengers movie. Now it looks like Thanos had another reason to use Odin's adopted son in his quest for the Infinity Stones.
In Avengers, Thanos ordered the Chitauri to aid Loki in procuring the Space Stone. So why not just get the Chitauri to finish the job? The new Marvel theory posted on Reddit suggests that the Mad Titan needed the God of Mischief for a different purpose that had little to do with the Tesseract.
The Mad Titan may have also been counting on the possibility that Loki would take the Tesseract before Asgard was destroyed. Although he realized that Thor would still be strong enough to defeat him, Thanos fixed this problem by killing Loki and hopefully leaving the Odinson devastated.
It's certainly a plausible theory since Thanos didn't seem too upset about Loki's capture in the first Avengers film. Unfortunately, it also means that Loki spent most of his life being manipulated by the Mad Titan.
Thanos returns in Avengers: Endgame, which premieres on April 26, 2019.
Related: Marvel Confirms That Loki Was Mind Controlled During First Avengers Movie