I kill it to gain XP is a difficult mentality to escape. It is also necessary if table top roleplaying is going to survive.
Firstly, make it perfectly clear that killing people doesn't net them karma. Say it three times a game until it sinks in. At the end of the game, tell them the things they earned Karma for. Rather than say "Y'all get 5 Karma." say "Y'all get one karma for playing, one for figuring out where the Corporate CEO lived, one for infiltrating the compound, one for roleplaying well, etc..."
Next, use the Notoriety system. It is one of the most versatile tools of the GM and one of the most underutilized. They do something cool, like make contacts in the barrens to discover info rather than torturing the next guy they see, they gain Street Cred. They do something uncool, like kill an NPC for no reason or take the last beer without asking, they gain Notoriety.
I like to make a good chunk of the group's income come from the Negotiation roll with the Johnson. This emphasizes good roleplaying and gives a monetary reward for good Street Cred.
Lastly, once you have all your skinner box carrots and sticks in place, just be impartial. A lot of roleplayers want to hack and smash things to show that they can, to throw wrench into what they feel is the GMs attempt to railroad them into how to play a character. To get a rise out of him. You can't do that. What they say they do, they do. You simply have to create the consequences of that action, and you need to do it fairly and within reason.
One of my favorite sayings is "The GM doesn't hate you, but he does represent people who do."
In my experience, once a player says "I throw the baby out the window." and the GM's response is with neither horror, disgust or incredulousness, but rather "Okay. It falls." the bloom is off the rose and the actual role playing can begin.
