I imagine Sniper thinks of himself as mostly an ambush predator - don't do too much that gives you away, by the time prey knows you're there it's too late to even fight back. Makes it more likely to be successful in the hunt. But Sniper longs to get more hands on, to sink his teeth into living, bleeding flesh and take down his victims more personally. And every so often, that urge wins out and the most stealth he has is sneaking up on someone, or striking when they think they're safe and talking to him. He finds it a relief to be more brutal, feeling more powerful and like he can do anything. It's addicting and I imagine after the first time it happens (perhaps accidentally) he just wants more.

Remember that line about feelings in Meet The Sniper? "Feelings? You know who has a lotta feelings? Blokes who beat their wives with a gold trophy, that's who." Implying he's not like that, but oh that was way too specific of him.