In a manner of speaking. The demons responsible for that were not the sentient sort. All they did was exist -- it's not as though they consciously chose to be demons who breathe toxic miasma. And even we didn't know the effects would be contagious.

[Not to make himself sound more innocent or anything -- but it's not like the complete eradication of humanity was ever their goal.]

As time wore on, it became clear to some of us that humanity would soon be wiped out if nothing was done. And so, demonkind split off into two groups. Those who supported Lucifer and allied with demonkind, and those who supported myself, and allied with humanity.

It was necessary, you see-- humans are our vessels. If they were to die out, we would have no means of crossing between planes and entering the physical world of Assiah. And all that aside-- your kind was simply far too interesting to be allowed to disappear over a simple temper tantrum.

[This still doesn't make him sound very nice. But just because he wasn't sympathetic towards humanity doesn't mean he couldn't become their ally.]

We provided humanity with the knowledge necessary to fight back against demons-- the power of exorcism. And eventually, the world reached a sort of equilibrium. As demons became less of a problem in daily life, less and less humans were able to see them -- and in modern times, many doubt we even exist.

[He's still leaving out a pretty big part on why ol' Lucy was such a sensitive subject...]