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Attacking people with a knife seems to show they're at least not mentally well. Although the mental illness excuse is only allowed for white people I think.
There's a risk of stigmatising people with mental illness as prone to violence, which is an inaccurate portrayal. However, if an offence was a direct result of the perpetrator's mental illness, then they get dealt with under Part 4 of the Mental Health Act. This usually involves compulsory treatment in secure psychiatric hospital, followed by very long term community supervision. It is very far from 'an excuse'.
That may be the case, but I've not seen anything suggesting he was mentally ill. (I might have missed something).