User blog comment:B214/Question and Answers 6/@comment-185.112.105.215-20161128063206/@comment-9595671-20161129045200

I think maybe at their prime, even Indra isn't strong enough, considering all factions took sides in the war that means even the Hindu faction did. The two aren't dead, so Indra likely couldn't kill either of them.

The possessors of Ddraig and Albion on the other hand? None have matched the two dragons at their prime. So Indra's warning likely relates to getting too close to his own level.

Unfortunately he's also unaware that Ophis is with him and has slowly begun to see Issei as a father figure. While Issei told her not to fight, that's probably out the window if he's in danger.