Definitely not on a goodie, and really questionable on a neutral.
The initial fight was fine, but to leave, wait 5-6 hours and come back when only priests are attacking the Archmage in the Chasm? The first mage is gone. That fight is over.
Conclusion: Pretty shady.
Suggestion: Imagine you're a wood-elf and a druid is fighting one group of very evil, nature-destroying mages (Scions) with the help of a nature-oriented mage. Sure, you have your reasons for not liking either the mage nor the Scions, and you might even get hot and angry in the middle of the fight if the druid interfered. But after the Outlander mage leaves, wouldn't you leave the druid to redeem himself by killing the Archmage? The only way I can see going back in is if you were an evil character for whom selfish desires and a wounded pride drove you to hurt bystanders when there was no point to it.
- Paul