"His lance blessed by the passion of the unjustly slaughtered"

I think @smithkm is on the right path with "unjustly," but "dead" is too passive and neutral. "Slain" is more poetic, but "slaughtered" really makes the wrongness of the deaths vivid and immediate.

I also would tweak "filled with the souls," which to me suggests that he was the killer, and that the lance drank the victim's souls. My first thought was "powered by," but that's a little bit comic-book superhero-y. "Blessed by the passion" better conveys the idea that it's the thirst for righteousness of the dead that is granting him power.

I think @smithkm is on the right path with "unjustly," but "dead" is too passive and neutral. "Slain" is more poetic, but "slaughtered" really makes the wrongness of the deaths vivid and immediate.

He wields his godly lance, the lance full of souls of the unjustly slaughtered

However, this makes it sound like he's killing people unjustly with his lance, and that it's then drinking their souls which doesn't sound like the hero you were describing. So, assuming he's fighting on behalf of the unjustly slaughtered (and not the one unjustly slaughtering them) I might tweak this a bit more:

"His lance blessed by the passion of the unjustly slaughtered"