How do you get the value after the two dots? (Line 7)

You want to use String:Sub(beginning, end)

As this will automatically get the rest of the sentence.

To do this

So, for example. If I wanted to get the rest of the name that a player said I would do something like this.

game.Players.PlayerAdded:Connect(function(player)
	player.Chatted:Connect(function(message)
		if message:sub(1,6) == ":kill " then
			local targetPlayer = message:sub(7)
			print(targetPlayer)
		end
	end)
end)

If I said :kill killerbrenden this will print killerbrenden.

The sub(1,6) is the length of the message :kill , sub also counts spaces and other characters.

So, for your code it would look something like this.

game.Players.PlayerAdded:Connect(function(player)
	player.Chatted:Connect(function(message)
		if message:sub(1,6) == ":kill " then --// Kills another player
			if game:GetService("Players"):FindFirstChild(message:sub(7)) then
				local targetPlayer = game:GetService("Players"):FindFirstChild(message:sub(7))
				local targetCharacter = targetPlayer.Character or targetPlayer.CharacterAdded:Wait()
				if targetCharacter:FindFirstChild("Humanoid") then
					local targHum = targetCharacter:FindFirstChild("Humanoid")
					targHum:TakeDamage(targHum.MaxHealth)
				end
			end
		elseif message:sub(1,5) == ":kill" then --// They kill their self.
			local targetPlayer = player
			local targetCharacter = player.Character or player.CharacterAdded:Wait()
			if targetCharacter:FindFirstChild("Humanoid") then
				local targHum = targetCharacter:FindFirstChild("Humanoid")
				targHum:TakeDamage(targHum.MaxHealth)
			end
		end
	end)
end)

The second part is if the player says :kill then they will die instead of another player, but if they say :kill Player_Name then that targeted player will die instead of the speaker.

Hope this helped! If this worked, don't forget to select this as the answer!