It's actually even worse than just being a "violent lunatic". He was a violent lunatic who had been arrested 42 separate times, 4 of them being for assaulting random people, the most recent being an attack on a 67 year old woman:

The woman sustained a broken nose, a fractured orbital bone, and "bruising, swelling and substantial pain to the back of her head" in the Nov. 12 attack, according to a criminal complaint.

This guy was 100% a danger to everyone in that city, and Daniel Penny did absolutely nothing wrong in acting on this asshole bursting into a train, and informing the riders that he was going to "fucking kill everyone".

It's absolutely disgusting that Penny is probably at greater than 50/50 odds for going to prison for stepping up, and stopping this menace in his latest (and final. LOL) attack. When you've violently assaulted multiple random people, up to and including literally breaking the faces of nearly 70 year old women, I don't really give a fuck if you die when someone tries to stop you.