Mike got frustrated because for two fights Holyfield kept head butting him coming inside and Mills Lane wasn't doing shyt about it...
He suiffered a nasty cut in the second fight from a headbutt right before he bit Holyfield...
People not involved in boxing intrinsically don't understand the conventions that you have to abide by to keep it a professional bout.
If you headbut, low blow, kick, elbow, grab the back of the head, kidney punch etc then it falls out of the agreed boundaries that keep it a clean fight aka the rules.
Holyfield head butted at least 6-10 times before Mike bit the ear the first time. Holyfield took it there, he made it a street fight. In boxing if you take it there then, conventionally, anything goes.
Mike went too far, but he's unfairly crucified for his actions imo. He was stupid, but he was enraged, and rightly so, Holyfield had pulled sucker shyt.
Similarly, Maweather got crucified against Ortiz ko sucker punch. Ortiz had taken it there with the massive headbutt earlier.
If you flagrantly foul in boxing then it isn't a professional fight anymore, it becomes a scrap. People don't understand that.