In the
Other World,
Goku is taught this move from
King Kai.
[14] Essentially, to use the Spirit Bomb, one must have a pure heart so they can manipulate and gather energy, otherwise the move can backfire and possibly hurt or kill the user. Likewise, once a Spirit Bomb has been fired, it is possible for one with a pure heart to deflect the technique, as Goku clarifies to
Gohanand
Krillin. However, a strong enough person can deflect it regardless of their morality; as shown when
Kid Buu, who is stated to be pure evil, resists the
Super Spirit Bomb fired by Goku during their final battle on the
Sacred World of the Kai.
I mean this basically implies it’s super effective on evil people and that it’s not just a big ki blast.