Say, How do I get the name of a killer of a player?

I know that roblox has a system with their weapons and leaderboard that when a player is killed a value is put inside the victim's humanoid and when the leaderboard finds the value (Which is an ObjectValue) it looks in that value and finds the killer's player object. You can take a closer look in Roblox's Sets in the toolbox and in their weapons. That's how I learned how to find the killer.

If that isn't enough then

--make a function that marks the victim like so
function TagHumanoid(Killer, Victim)
	local c = Instance.new("ObjectValue") --Create an ObjectValue
	c.Name = "creator" --Name it creator
	c.Value = Killer --The Killer's player is stored in the value
	game.Debris:AddItem(c, 5) --The value will be deleted in 5 seconds
	c.Parent = Victim.Character.Humanoid --The value will be placed in the Victim's Humanoid
end

Here's how I get the victim's player through a sword LOCALSCRIPT:

local Player = game.Players.LocalPlayer

Handle.Touched:connect(function(hit) --Spawn a touched event
	hitplr = game.Players:FindFirstChild(hit.Parent.Name) --Search for hitplr
	if hitplr then --If it exsists then
		hitplr.Character.Humanoid:TakeDamage(damage) --hitplr takes damage
		TagHumanoid(Player, hitplr) --HAHA I TAGGED YOU!
	end
end)

If you have a gun with a moving bullet (one like manofthelol's guns) then it should work too.