@BalrogDeMorgoth while that is true, and he does get a kick out of it, he isn't exactly just your average serial killer. His enjoyment comes from his understanding of justice, wich can kinda be TL;DR-ed into "society's justice didn't save my loved ones, so i will save others, and correct justice's mistakes", and his enacting of it.
He genuinely believes what he does is not necessarily good, but "justice" and he feels ecstasic when it finally happens.
What he's doing is basically just the extreme version of the average "good" samaritan, where they secretly celebrate the deaths of the corrupt and the guilty, except here he's the one doing the killing and celebrating very personally, wich imo can't be compared to regular ol' serial killing, ofc the morality of it is a whole other issue.