When I go to the nether, I get attacked by neutral zombie pigmen. I feel like it would make more sense if neutral mobs didn't attack unless they were aggroed. If this is an intended feature, a config option to turn it off would be nice.

Note that turning this off just for zombie pigmen isn't a great solution, because you would lose out on the added risk that you get if you aggro them.

Would an option to make pigman always hostile be possible?

That's not in the scope of my mod, but i wouldn't be surprised if there is a mod out there doing exactly this.

That's a quirk of the minecraft engine, or rather, how Pigmen are implemented. The AI is constantly "targeting" the player (which Infernal Mobs reacts to) but a flag inside the entity update itself aborts the attack every tick unless the player has aggroed them.

I can think of solutions - such as making sure an entity was "targeting" a player uninterrupted for several ticks before triggering mods - but it's a bit of a pain to do this to fix what is essentially an engine error.

I suggest to just add pigmen to the Infernal Mobs blacklist.

Quick note: This is in the 1.10 verison.