I think he is using the "angel of death" angle.
I have had the opportunity to work on weapons for military snipers, and some of them like to use names like this.
One used the call sign "Archangel", 2 others were "Stormbringer" and "Mournblade" (a team, names taken from Moorcock's Elric series, they were the names of 2 soul sucking swords), another was "Balthus", (a hero in a Conan novel who fought off a pictish horde), another was Jacob McCandles (even though John Wayne wasn't a sniper in the movie), Another was "Joe Kidd" (after the Eastwood movie of the same name), etc. etc, etc. BTW as far as I know these guys are long retired so I'm not giving up any secrets
The names they pick have something to do with their egos, and if this sniper is leaving "death" cards or associating himself with the Archangel Michael, he is trying to portray himself as the same calibre of man, even if he isn't.