I love how a lot of videos on Arlecchino try to justify his actions to make him out to be a "misunderstood villain" or " victim of circumstance" but in reality.... he's just evil

If Arlecchino were to redeem/reform himself in some type of another life. it would be on a prayer and an act of God.

Wasn't he already a serial killer? It's mentioned that the ego of a serial killer awakened within him, I always interpreted that as the Workshop Union and Alchemists accidentally putting Ergo from a petrified serial killer into a butler puppet.
It's also pretty clear from Arlecchino's actions that he's simply depraved. He enjoys the pain and suffering he inflicts on others and will happily rob others of their lives just to make his 'art'. For all we know, he was already doing a smaller-scale version of this kind of thing when he was a human.
The only tragic thing about Arlecchino is that people with mental health issues, the small percentage that become serial killers due to them, only do so because they aren't given the help they needed. Arlecchino though, in all honesty, seems like one of the people who uses mental health as an excuse to do these things, similar to Obake from Big Hero 6, who knows he can't tell right from wrong, knows he could get that fixed, and chooses not to specifically so that he can do his plan without having second thoughts.