If he did it judiciously, which I have no way of knowing, then I don't have a problem with him preemptively attacking people who are killing that mob. Read what he wrote:

<i>I did not use the Quaggoth to kill paladins. I used it as a location to kill my enemies who were getting its blood to come kill me. If there were individuals who were not enemies of the village, or helping them, they killed the Quaggoth fine, actually while I watched. But these supposedly innocent paladins would kill it for their mage buddies, they would also kill it and then come behind us while raiding Nexus.</i>

Conditions where he'd kill people who were killing that mob:

1. Mage.
2. Enemy of the village.
3. Helping an enemy of the village kill the mob.
4. Helping a mage kill the mob.

If a paladin (or fortress guy in general) makes a point of going after an evil villager then I have zero problem with that villager going on the offensive against that person.