Currently, respawn level behave strangely. When a level is respawn, all death
bot in level are resurrected, even bot from another level. Likewise, bot dead in
an another level don't respawn in origin level. As a result, bots migrated
between levels. 

There is problematic in difficult level with few bots.

This isn't a bug - what you're describing is the way level respawning works.

Please explain how you think it should behave.

Some time ago we were respawning bots at the very position they died at,
regardless of which level they came from.

This caused a lot of trouble because with time, there were going to be more and
more hostile bots around friendly npcs (not saying "friendly enemy" here because
I think it sounds stupid, sorry Arthur :p ) and the bots would have killed them
immediately.

I think, because respawn is rarely used, we can killed all bots in level then we
respawn bots with level specification. Respawning bot at the same level they
died at are not a good idea.

The lost of performance and memory is insignificant with the delete all bots in
level.

Also, currently, only human npcs are not respawned. But, droid npcs exists and
can be respawned (and might break logic).