You are correct, although planned out revenge killings are not considered psychopathic.
Killing horrible people, in spite of the grave personal consequences, for no other reason than the pursuit of actual justice, that I have no problem with. Call it an honor killing or whatever, not all life is sacred. It violates the law but it doesn't trouble me, not in this instance.
Two birds with one stone? If Shenks truly has lost any sense of purpose and no longer desires to continue living, the noblest and most honorable thing he could do would be to find these guys and end them. Few people ever so much as glimpse an actual moment where the cosmos itself hands down purpose. Acting as a divine agent of karmic retribution validates everything in your life prior to that moment. Wiping this scum out would elevate Shenks to some sort of Bodhisattva-type figure. Not condoning it or planting the seed here, real talk, just positing some thoughts.