I haven't read or heard any direct quotes from his parents. They obviously struggled with his behaviour, but it appears the heaviest 'punishments' for carrying knives into school were being banned or suspended from schools. The government scheme called Prevent can also be apportioned some blame as they reportedly 'intervened' at least three times over the years. He was on a watch list, but they still managed to miss opportunities to at least have him sectioned.
His father stopped him getting in a taxi to go to his old school where he claimed to have been bullied and, armed with a knife, was going to kill the bullies. The following week, it appears he was going to walk to the school, but then went into the dance class and committed the assaults and murders. It was almost like a secondary target.