So, purple-ish with an alarm happens around your slaves/workers, they spawn sleeping and eventually wake up. Though I never experienced any damage from those.
Ninjas, though, have an invisible unit which drops a portable nuke on becoming unconscious/dead, and this explosion can wipe out the room easily. The only way to prevent it from explosion is to pick the bomb up (and you can only hold it in hands, can't disarm either). I think on the second difficulty there are 3-4 of those suicide bombers