why do so many assume he suffered from mental illness? Sounds to me like he was a drug addict from a broken home with little or no family support or parental guidence.
As someone who's had to deal with the effects of addiction, this is an ignorant statement, and is unfortunately the baseline way of thinking for most out there. One thing to consider; no-one, at any point in their life will wake up in the morning and say to themselves, 'I want to be an addict'. There's no glamour or positive outcome to the life of an addict, addiction is a mental disease. It can come from a series of events and bad decisions, and will trap you and enslave you, the only thing an addict cares about is getting the next hit. Petty crime is normal; stealing to get money for the addiction, murder is not normal, it doesn't help the addiction in any way.
Also, if someone is from a broken home and that has driven them to murder, then how is that not mental illness?