If you want to prevent any accusations of RDM, change your initial plan to a ''grab'' instead of a kill. Take some zipties, find his location (in rp, talk to people etc.), make him surrender and capture him, take him somewhere remote, tell him you were hired to kill him and do it. Because of you taking him hostage, basicly, you initate roleplay, and by talking to him before killing him you prevent any randomness involving the kill.
This is also way more fun for you AND the target than just warn him and snipe him 5 minutes later =)