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).