User blog comment:Piyush Banerjee/Shin DxD 2 discussion/@comment-2607:FB90:4A8E:C500:80A:87B2:1048:97F3-20190609012154/@comment-10330822-20190609024416

In truth, it's all about the power balance. Indra knew that the Hindu mythology would be in chaos after the fight with Trihexa, so he planned to attack Shiva to help establish a proper hierarchy among the remaining gods.

It makes more sense when you consider Indra is a battle maniac. He's not really evil, he's the Hindu God of War.

When he helps Issei to become strong, its to make a more interesting fight for himself. Of course, we all know its going to blow up in his face, but that against the point. Indra isn't solely in the tournament to get his war with Shiva, he wants to have a good time in the process.

Also, I don't think he is going to win, he's just a favorite right now. Issei probably will win so he can make good on his promise to Tiamat. Plus its highly likely that someone on Issei's team will make the wish for Dulio and Rudiger's team, because it is a worthy cause. And there might also be a wish for Satan's having harems, as the Underworld wants to make Issei a Satan, but he can't have his harem if he becomes a Satan, so unless the rule changes that will be a problem.