What about gravity assists, harvesting kinetic energy from other planets? It would make for a good supervillain story, intentionally ramming the Earth with a g

What about gravity assists, harvesting kinetic energy from other planets? It would make for a good supervillain story, intentionally ramming the Earth with a gravity assisted projectile, but a terrible weapon system. Any conflict likely being already over by the time it does its tour of the solar system and arrives back, so it's probably better to just use the rocket fuel to build a conventional bomb. Or better yet, use nukes.