I never thought I'd say this, but maybe it's time Damon be a bit less trusting. Instead of immediately sending Kai back to Hell after his surprise rise from the dead on The Vampire Diaries, Damon made a deal with Kai in "The Lies Will Catch Up To You." The deal was simple: find a way to bring Kai back from the dead permanently (turns out, his return is only temporary, and he's kind of living between dimensions right now), and Kai will bring back Elena and help them defeat Cade. Of course, this being Kai, he stabbed Damon in the back and took Elena's body instead. So, why did Kai take Elena's body on The Vampire Diaries? Well, for one thing, he's evil.
Kai not only used his magic to take Elena's body, he also siphoned off energy from Damon to do it, leaving him helpless to save her (at least, for now). Damon tried to convince Kai that the only way he would be able to fully escape going back to Hell would be to redeem himself before succumbing to his own death. And what better way to redemption than helping Damon get his Happily Ever After with Elena? Unfortunately for Damon, Kai had a different plan in mind.
After telling Damon that redemption was just a foolish dream people destined for Hell believe in, he vanished with Elena, and swore that if he was going down, he'd do it with a bang. There could be a million things he has planned with Elena's body. But, as always, it seems like the most logical thing would be to use Elena as leverage. Whether it's to ensure Damon kills Cade and get shim out of Hell, or to make a deal with Cade for his freedom, whatever Kai has planned with Elena, it can't be good.