A couple of reasons why you got killed by one and are now coming to the forums to complain about them:
1. You are bad at the game and don't want to form a way to counter them.
2. The warlock who killed you plays their class very well.
3. Coming to the forums to cry for nerfs just proves that you are bad.
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I have played regularly as all three classes, and i still feel that warlocks are slightly OP. Not enough to say the character needs a nerf, but they are noticeably more effective than any other character. They have much better grenade choices and the warlock melee range.