Cool. As you get time put together your charcter and we can take it from there.

My preference would be to have you perform an initial freelance mission we can both use as a test run. Ideally I would also like you to do at least one contracted mission for the players before turning against them (it's just so much more cinematic).
As you make your character, keep in mind the following:
1) Any potential abilities that could help you set an ambush, and any abilities that give you a strong showing in the surprise round and the opening round.
2) Abilities that help you escape after the attack - Athletics, Blend, Drive, Falsify(Disguise), Sneak, etc - and related feats and class abilities. You don't need all of them, but your hitman should have a primary escape ability and ideally at least one backup one.
3) The target will have a (rolled) vitality level equivalent to a level 3-6 player character. The longer the combat takes the more (chance and quantity) law enforcement or security minions will become involved.
I figure a posting cycle something like this.
GM - Post a mission brief
Player - Post selected mission gear choice (normally Caliber III)
GM - Post scene description
Player - Post preperation for ambush and overall plan
GM - Resolve ambush round
Player - post intended action for next combat round
GM - Resolve above
(possibly repeat above 2, maybe a few times)
Player - post intended action for escaping crime scene
GM - Resolve above
(possibly repeat above 2, maybe a few times)
GM - Mission completed, failed or reattempted.
A bodyguard mission would probably follow a similar structure, with resisting an ambush attempt and getting the client to safety.