Not necessarily... It never shows Locke pull the trigger. I heard a pretty interesting theory earlier, I'll see If I can remember it: Locke finds chief on the Ground, like in the trailer. Notice how it looks like chief has plasma burns- Locke has no plasma weaponry. Locke pulls out his pistol... Ok this theory starts to get weird... He shoots the chief, intentionally leaving him alive, but he looks dead to ONI. Hence the crack in his visor in the original trailer where we see him cloaked... Far fetched, but who knows?