Why do monsters attack certain targets?

[Wyrm]

Monsters don't really "lock onto a target" per se, afaik, but instead "commit to a current action".

That's more or less what I was trying to say, I didn't really make the distinction at the time. I was thinking of lock on more or less the same way a missile locks on a target. I did try to imply though that once a monster executes it's current action or is interrupted somehow, it's no longer "locked on".

despite the threat level factor, a Decoy won't pull a monster off of you without proximity working in your favor.

I guess I need to go into more detail when I explain things... Oh well. What I meant was that in my experience, monsters check the range of targets first, then, if there are several targets within the same general range, they'll go for the most threatening one first.

However "Attract" can

Well now, that's interesting. I always wondered what the radius effect on Attract was for, I always just assumed that if you didn't target a specific monster, then it chose a random enemy within the attract range.

When something like Holy Fire touches a monster (high range Static Field, etc. etc.) it may very well be "waking them up" from the "idle" state.

That's exactly what I believe, and fits in with how the thing works. Those pulses are attacks and monsters don't like being attacked. When you use a Fire Golem, or even have a really long range aura (Sanctuary, anyone?) you'll be hitting monsters well outside their visual/noticing/whatever range. They then become instantly aware of the attacker and beeline at them. Sometimes this can work to your benefit when a merc/minion is hit by the pulse from an Aura Enchanted (Fire, Freeze, Shock) boss that has a reviving ability (shamans, etc). Although if the aura is Holy Freeze, you're more screwed than saved.

Depending on whether this implementation exists at all, it is interesting to wonder if the "slot" can be overwritten (e.g. perhaps by a higher net damage attack?)

Interesting concept, but I don't think monsters keep track of how much damage they're dealt, just whether or not it's dealt. The reason I say this is because a high level Valk can have damage all over the scale in Hell, from barely noticable to near one-hit-kill. When both she and my Holy Freeze merc are attacking the same boss enemy, the boss usually won't switch targets once it starts attacking, although for some reason if they start out targetting my merc and I move him away from the boss so the Valk is the only thing to target, then let him get back close again, the boss will retarget the merc. This ties in with what CelticHound pointed out, meaning monsters may actually have a hate/really hate list).

Converted monsters are interesting as well (no time to delve into that now... did you know that even though you can't target them they will still die from having been previously poisoned by now "friendly" sources).

I don't really bother too much with converting monsters except for the occasional unintended one from Mind Blast, but that would make sense considering you'll still take poison damage in town, and mercs/minions can still die of poisoning in town (and you probably could too if Blizzard didn't have the poison duration 1hp limit for players). I'm a bit surprised no one's ever made a build around high poison damage and conversion, actually...