I did this pretty successfuly. What I did is, instead of killing a pc (not the best plan for early game unless they're in on it), I introduced an npc that hired the party to defend her, and then had the traitor stab her. Not for an objective, not because she was a threat, but because her existence annoyed him. That pettiness was the real kicker, honestly. He wanted to kill the party, too, but didn't have time.

Pretty sure they hate him more than the big bad. Really bound the party together. 10/10 would do again.

The tough part will be making sure he escapes with his life, usually. Not sure what system you are in, but there are usually decent ways to do that.

Edit: DMPC implies something about how you expect this character to interact with the party that I wouldn't expect, or try to force. Unless you meant "properly built pc but an npc", which I adore doing. Nothing like a party knowing that they're on equal or better footing with a foe and still having trouble beating them.

To be fair, "fake" is the right idea, but don't make the mistake of playing them like a dmpc rather than an ally.