Should humanity construct an interstellar doomsday weapon? Hear me out.

We could construct a missile with a cobalt bomb, capable of destroying all multicellular life on a planet. This missile could be parked in orbit around Pluto, or any other hidden place in the Solar System.

If aliens were to attack us, this missile would be launched toward their home planet. Over decades, it would gain speed and eventually reach few percent of light speed. It would then impact their home world with enough kinetic energy and radiation to destroy all life.

The missile would have to be "camouflaged" and would emit no radio signals at all. It would try to get as close as possible to their home planet without being detected. If countermeasures were to be employed, it would perform evasive maneuvers, but after decades of travel, the speed would probably be too high for any real countermeasures.

Is this idea feasible and should we do it?