Yes, already noticed this, and firendly NPCs won't see what's outside their
level, so letting bots respawn on the border is also safe.

Engel and Geist are most likely to be killed, and the only ones that teach
extracting bot parts, and you need all available levels in order to be able to
produce all item enhancements.

Francis isn't much of a problem, because you get his quest first, but Kevin
gives quests and teaches hacking (which in theory could be acquired also by
excessive nethack playing).