I think DtyCln is closer to the mark, for coalface ATCOs at least.
Proving who pulled the trigger will be straight forward, proving who loaded the gun, rather less so. In the event of a collision, the moment when a potentially dangerous situation turned into a lethal one would probably be obvious, particualy if it hinged on an ATCO issuing an executive instruction. The onus would then fall to the ATCO to prove that the instruction he issued was the correct one, given the information available to him/her at the time, prevelent conditions, established procedures and level of training.