User blog comment:B214/Volume 22 spoilers/@comment-74.173.162.168-20161030032159/@comment-9595671-20161030213929

I think that's a secondary reason Shiva's began showing an interest in Issei too, Indra wants to go to war with Shiva, but he's not crazy to try much without a strong enough army.

If Indra's aware Shiva's considering Issei as an ally to him, Indra is likely wise enough to wait until his war begins before he tries, luckily Ajuka and Shiva seem to be taking Sirzechs and Azazel's rule to a degree.

EX seems to even imply he'll eventually have to accept Issei's the only reason the world's able to stay safe or his war failed and Shiva's interest means he can't target Issei at all without risk, and I think he'd go far enough to kill Indra.