Answer by taking a quote from the following article:

Keane was born into a working class family in Ballinderry Park, Mayfield, Cork. His father, Maurice, took work wherever he could find, which led to jobs at a local knitwear company and at Murphy's Irish Stout brewery, among others. His family were keen on sport, football especially, and many of his relatives had played for junior clubs in Cork, including Rockmount. Keane took up boxing at the age of nine and trained for a number of years, winning all of his four bouts in the novice league.

Keane made headlines again in the 2001 Manchester derby, when five minutes from the final whistle, he was sent off for a blatant knee-high foul on Alf-Inge Haland in what was seen by many as an act of revenge. He initially received a three-match suspension and a PS5,000 fine from The Football Association (FA), but further punishment was to follow after the release of Keane's autobiography in August 2002, in which he stated that he intended "to hurt" Haland. Keane's account of the incident was as follows:  I'd waited long enough. I fucking hit him hard. The ball was there (I think). Take that you cunt. And don't ever stand over me sneering about fake injuries.  An admission that the tackle was in fact a premeditated assault, it left the FA with no choice but to charge Keane with bringing the game into disrepute. He was banned for a further five matches and fined PS150,000 in the ensuing investigation. Despite widespread condemnation, he later maintained in an interview that he had no regrets about the incident: "My attitude was, fuck him. What goes around comes around. He got his just rewards. He fucked me over and my attitude is an eye for an eye", and said he would probably do the same thing again.  Haland later implied that the tackle effectively finished his playing career as he never played a full game afterwards. However, Haland did complete the match and played 68 minutes of the following game. He also played a friendly for Norway in between both matches. It was, in fact, a long-standing injury to his left knee that ended his career rather than his right.

Why was he angry with Haland?
He fucked me over and my attitude is an eye for an eye