I wouldn't be a bit surprised if the medication they put him on had something to do with it, some mind altering drugs in the past have proven to be the actual cause of similar massacres, E.g.
Paroxetine (brands names Paxil or Seraxat), for instance there was a man acquitted of killing his wife and kids while under the influence of this dangerous drug and there was numerous law suits in the USA. Yet it is still sometimes prescribed in the UK and I know how dangerous it is from how it adversely changed me when I was wrongfully prescribed it after the doctors stated that my untreated under-active thyroid + sleep apnea and the severe fatigue it caused was only depression until much later when the damage was done (I'd also lost my job by then). I also dangerously felt invincible while also violent despite being one of the most placid people you could meet normally, in fact it's a long story, but the drug was very detrimental to my life and I should never have been put on it, there was a solicitors interested in suing for people in the UK too that I got involved with, but unfortunately it never came to anything unlike the USA. I can therefore say for a fact that it is a very dangerous drug, but corrupt pharmaceutical companies will still continue to push it knowing damn well what it can do to people as well as many other dangerous and addictive drugs, they even sometimes "sponsor" the media to be biased (I read about this in a CNN story) and have very expensive lawyers to protect them while they make $Billions out of other people's misery.
As far as autism is concerned, it's unlikely to be directly the cause. It maybe indirectly since this may have caused him to end up being treated for mental health in the first place, often a slippery slope. People on the higher functioning end of the autistic spectrum know what is right or wrong in extreme circumstances such as killing, in fact I think even my brothers on the lower end of the autistic spectrum who can't count or tell the time would know it's wrong to physically hurt someone, although they may not understand that a gun kills and what killing actually means. In much lessor circumstances where there's grey areas some autistic people might not realise they're doing something wrong until they're informed when most NTs understand, but they're not likely to do anything purposely wrong any more often than an NT.
Edit: Obviously we still get good and bad people however that are both autistic and NT, with many you could argue that it's related to their upbringing and this maybe partly the case, but I also think that it's often at least partly built in the genes. I say this because I remember a neighbour of my parents who adopted a son from a baby who I watched grow up as I did, he came from a bad family background, but he was brought up in a proper loving and caring home, in fact his adopted parents were lovely and did everything they could for him. As he got older however he still started stealing and getting involved in other crime, he soon wanted to get involved in drugs including selling them and no matter how much his loving parents tried he ended up in and out of juvenile detention and then adult prison as he got older, I believe his parents were very sadly fighting a losing battle because it was in his genes, but perhaps he could have turned out even worse still without a good upbringing, a murderer even. This can in my opinion affect both autistic and NTs equally. That said there are examples of brothers or sisters where one turns out evil while another is good with exactly the same upbringing and parents, this implies that there are occasionally just evil souls.
Edit 2: It doesn't help when people are told that killing humans is wrong when they watch the news and hear about wars, killing and fighting all the time. It's also apparently okay for the leaders to order other people to kill while also financing armies and weapons to help them do this more efficiently. Then we all know that powerful governments assassinate people and then there's the death penalty that is "legal" murder in my opinion, apparently it's perfectly okay to kill then too. Now guns are mainly designed to kill, especially people, yet they are legal for many people to have in the USA, this is also contradictory and they're more likely to be easily accessable when someone "loses it" or has a breakdown. Killing is wrong FULL STOP, no exceptions.
