The act of killing a loved one or friend in order to trick your evil overlord into trusting you completely and or believing that you are their ultimate agent.
When you kill/betray person-A to trick person-B into thinking that you are loyal to him. But actually you just want to get closer to person-B to destroy him!
To "Snape Kill" means to kill your former friend/ally or comrade to prove to your loyalty to your dark master so you can continue to be a double agent for the other side