There's been a lot of excellent stuff on how to twist players by hitting them emotionally, so I'm going to suggest another way that can be used supplementarily or seperately - hit their characters in the wallet
Their fixer calls, saying he's got two possible jobs on the line. Would they be willing to put things to one side and kill? No? Ok, right, onto the job that will only pay half as much...
Then follow that with a job for a high-paying, good value client. Maybe give them two or three missions with this guy before pulling the stunt here, just to make clear that they're on a good meal ticket here. Things go smooth, they come back happy - and then when they go to collect their pay, he starts screaming at them because one of the guys they left alive had a copy of the research and what they've stolen for him is no longer worth nearly as much. He stiffs them and tells them to get out of his sight before he spreads the news about how much they ballsed up. Or maybe he just spreads that news anyway.
Maybe the job after that the Johnson lies and neglects to tell them it will mean going straight through a crowd of high armoured pain tolerance modified psychos.
Or maybe the key snitch will hand over the info after a bit of wetworks. Or their highly trusted, highly connected contact needs a cop taken out. Or maybe they've had no work for three weeks and the only thing on the table is killing.
And so on. You get the idea. Its great they want to be professional. But sometimes professional means killing. And sometimes, not killing, means not eating. Just a thought anyway.