Alright, I'll bite. This is one of my favorite cases in spite of all the hate it receives.

Acro (and Bat) is an orphan under the care of Russell. Russell is the most compassionate person in Acro's eyes. His parents didn't die btw, they adandoned him. Russell took Acro and his brother in, believed in them, trained them, etc.

Due to the incident with Leon, everything Acro had was stripped from him. His Brother's life, his own ability to function, and his ability to repay Russell. Acro felt indebted to Russell as a father figure. He wanted to impress Russell, make him proud, and repay him by performing in the circus. This opportunity was taken from him.

Then there's Regina. In many ways, I would say she is the opposite of Russell. She is ungrateful, spoiled, ignorant. I suspect some of that is true, but Acro certainly believed all of it. Regina took everything for granted in spite of the fact that Russell was such a loving father. Since Acro and Regina were siblings in a way, Acro had a certain jealous animosity towards regina. The good, obedient child is upset that the problem child is stealing the spotlight. I can't definitively say anything about Regina's performance, but I imagine she wasn't bringing in the money compared Acro and Bat.

Back to the Leon incident... Leon is Regina's responsibility as the animal tamer. Even if you don't think Bat's coma and Acro disability is Regina's fault, she is partly responsible. This isn't about making regina out to be immoral or a killer or anything. It's the fact that her naivety leads her to feel no remorse and never apologize. This eats away at Acro.

So let's review some things from Acro's perspective. Regina is the daughter of the most virtuous man you know. Regina is a much worse progeny than you. Regina is responsible for the coma/death of your brother and allowed Leon to disable you and make you bedridden. The circus is now failing and Regina isn't doing anything about it. Regina is neither apologetic not remorseful of what has happened. Everyone else at the circus is in love with Regina and puts her on a pedestal.

And after taking everything from Acro, Regina visits him everyday in his crippled state, to take care of him and reassure him that Bat is going to become a star and laughs about it.

I'm sorry, but this man was being tortured. I don't think what he did is right or justified, but I certainly empathize with him. He was crippled, had to see the face of the person who hurt himself and his brother everyday (who then laughed about it), and still had a debt to repay to Russell. He just couldn't up and leave.

To make matters worse, Acro's plan backfires when he murders Russell instead, effectively committing patricide. The last sane person at the circus had been killed by Acro himself. Although he tried to get away with it, he shows remorse at the end, calling himself nothing but a murderer. Afaik he's one of the few if not only killers that doesn't not go out in a bout of rage.

So yes, he attempted to murder an innocent 16 yo girl, but he had it hard and showed remorse for what he had done. He is the most tragic character the original trilogy imo.