They do check people, but somehow he managed to get in. This happens, sadly.
Bilingual 48's Twitter has translations of official news and such, if people are looking for a credible source. I follow the guy who runs it, and he's said he's never going to post false info. Speculations, yet, but he adds disclaimers and stuff.
That guy must've really not liked the group, or was just downright psycho.
I honestly don't think speculation on his motive is gonna help anything. Yes, he's fucked up, but chances are we're not gonna know why he did it for a while. Last I read, he's in police custody and is undergoing questioning.
Mini-rant that's not really aimed at anyone but is more of a general thing:
Also this is somewhat unrelated, but I don't really like it when people assume attackers/killers suffer from a mental illness. To me, that implies that people assuming that think that all people with "mental illnesses" (which is a super broad term, but you get my point) are capable of killing or injuring someone. Now, I'm not saying they're "normal" (for lack of a better word), but still. I, for instance, have what qualifies as a mental illness and yet I would never ever try to hurt someone on purpose. The thought of hurting anyone terrifies me.
Lots of things qualify as a "mental illness". Anything from anxiety to depression to bi-polar disorder. And every one of those things is different. Hell, two people who are diagnosed with the same "illness" can be completely different.
Obviously I'm not saying that some people who hurt/kill others don't suffer from some kind of "mental illness" -- I just personally think it's wrong to automatically assume it. Because, in my opinion, that says something about what those people think about people with a "mental illness". That, to them, people with "mental illnesses" are all capable of harming other people, no matter what. I know a lot of the time that isn't the case, but that's what I feel like it says.
TL;DR: Just because someone's in the state of mind to hurt/kill other people doesn't mean they have a mental illness. A lot of the time they're just massive assholes (which is an understatement, really).
It's probably not very coherent because it's 2AM, but I felt like ranting. This happened hours ago and I'm still annoyed. It's not because of anyone here, it's more because of the media in general, I guess.