Killer = script.Parent Killer.Touched:Connect(function() game.Players.FindFirstChild.Humanoid.Health = 0 end)
First of all you'll need to gain access to the player's character in order to take their health away like so(I'll assume its a normal script):
Killer = script.Parent Killer.Touched:Connect(function(hit) -- hit will be whatever has touched the part (e.g left leg, etc) if hit.Parent:FindFirstChild("Humanoid") then -- we need to make sure it was a character hit.Parent:FindFirstChild("Humanoid").Health = hit.Parent:FindFirstChild("Humanoid").Health - 100 -- Takes all their health. end end)
Any errors will likely be on your end, e.g not placing the script in the appropriate place.