How would i tell a player who has killed them?

as gecko said, make a Script that Reads the Value, but by Default, Roblox Killing Tools have creator Values that are ClassNames ObjectValue and the creator of the Bullet or other stuff, is set inside the Value, so all u gotta do is just do creator.Value (cos Object Values are already set to the creator) and that leads to the creator, take the creator's name, and change the Text to the creator's Name...

Well if you make it so that when you get damaged by a player's weapon they get a IntValue with the damager's name in the value, then all you need to do is create a script that reads the value, creates a GUI on death and changes the text in the GUI to the name in the value.