Look, Endeavor fucked up his family, I can acknowledge that. People can still be mad at what he’s done even if he is attempting to atone because expecting people to forgive an abuser for decades of shit at the drop of a hat is unreasonable.
What I can’t acknowledge is using it as a justification for literal evil. Dabi was messed up by Endeavor’s poor parenting and worse handling of a delicate situation. Plead insanity and blame his upbringing all you want, but it only goes so far.
Grudge against Endeavor: Fine.
Use grudge as an excuse to hurt everyone close to Endeavor: NOT Fine.
Commiting mass murder in the name of your revenge: WHY IS THIS EVEN AN ARGUMENT?!
Personally, the moment Dabi started mass burning people for whatever reason, even simple as getting in his way if the alley scene is anything to go off of, he stopped being a character I could sympathize with because he’s doing horrible stuff, fully aware he’s doing horrible stuff, and doesnt give a damn who he turns to ash in the crossfire.