Nah the dark forest doesn't really work, If turning on a light (so to speak) makes you a target then a muzzle flash is even worse. It takes a lot of energy to kill a planet however you do it and thats going to tell everyone where the shooter is.
And no you can't use an asteroid because all the matter in the universe couldn't make a computer powerful enough to make it hit over a reasonable distance and getting to our solar system to use one of the ones here is just as energetic as firing a projectile.