The Merciless doesnt activate its exotic perk, what would make it firing faster on non lethal hits until something gets killed (it happens sometimes but maybe once every 30 enemy).
And a little offtopic it could get a little love, to make it a good option again but the fix would be fine too.
-
Maybe you should check out the forum I just wrote it's about the exact same problem and I did some more testing, maybe you can back me up..