archviles always respond to infighting

Unlike most monsters who seem to ignore the damage dealt by other monsters if the player attacks first, archviles appear to always retaliate against whomever hits them last. It is funny in situations with an arch vile surrounded by a bunch of bullet spraying chaingun guys to see it quickly change targets. What is it that makes arch viles so finicky? Does it have to do with all the states it uses to attack?

No, it's hardcoded behaviour of the thing type archvile. (In ZDoom, it's represented by so-called QUICKTORETALIATE flag.) As well as the other archvile's special ability, that he cannot be targeted by other monsters. (NOTARGET)

There's a mechanic in the game called a targeting threshold. When a monster obtains a new target there's a cooldown of about 100 tics wherein the monster is not allowed to change targets again unless his current target dies or this threshold times out. The arch-vile is specially hardcoded to never have a targeting threshold.

Melon said:

There's a mechanic in the game called a targeting threshold. When a monster obtains a new target there's a cooldown of about 100 tics wherein the monster is not allowed to change targets again unless his current target dies or this threshold times out. The arch-vile is specially hardcoded to never have a targeting threshold.

Oh wow, I always thought it had something to do with the amount of damage being dealt. Something like the second target having to do twice as much damage as the original target to get the monsters attention, with the player being able to override it or something.

This has been explained on the wiki for a long while:
http://doomwiki.org/wiki/Monster_behavior

Well, the same happens with lost souls, doesn't it?

Fernito said:

Well, the same happens with lost souls, doesn't it?

Not exactly. While Lost Souls do change their target when damaged, they will only retaliate once, unless damaged again, unlike the arch-vile, that will continue its revenge until the target is death (unless, of course, it's damaged again by another monster).

Zed said:

Not exactly. While Lost Souls do change their target when damaged, they will only retaliate once, unless damaged again, unlike the arch-vile, that will continue its revenge until the target is death (unless, of course, it's damaged again by another monster).

Right! Forgot about that.

If the arch-vile didn't infight immediately, the number of UV-Max demos of Hell Revealed 2 on YouTube would be reduced by 90%.

It's not a good UV-Max run unless the player, summoning the jammiest luck in the universe, survives incineration thanks to the interference of a Hell Knight right at the last moment.