I think it should be both, depending how long you hold the trigger for. As you charge it up, a gauge shows you it's strength, from "low", to "stun" and to "lethal". Too much and it'll kill (useful when you actually want to kill, you just use it how it works now). Too little and it does nothing but hurt them, knocking off a little health. Somewhere in the middle is the right amount to stun.The zapper should be a non lethal alternative to dealing with scientists. Why no "game designer" thought of this obvious mechanic is baffling.
My personal take on it anyway.