For all intents and purposes, you can consider fire giants, drow and duergar to be in the same class of being as the undead. That is to say they're categorically evil and everybody in Thera knows it.

Also, good-aligned characters are presumably not killing evil beings purely for personal gain. From an OOC perspective both groups kill to get XP. From an IC perspective good-aligned characters should be killing evil beings to prevent them harming innocents, to cleanse Thera of the creatures' taint, etc. etc.