Perhaps this is common knowledge, but if you use a tranquilizer gun and shoot an enemy in the head it will kill them. The stupid thing about this is you will likely not notice, as it still gives you XP for a "merciful" takedown. I'm not sure if it's a glitch or intended, but be aware of this!
edit: you can tell if an enemy is dead or not by looking at their body. If there is a skull icon, they are dead. If there is instead a "Zzz" icon, they are unconscious.
edit: you can tell if an enemy is dead or not by looking at their body. If there is a skull icon, they are dead. If there is instead a "Zzz" icon, they are unconscious.