People are inherently evil. They need someone or something to keep them inline, otherwise, they'll do whatever they want. Everyone here has proof of this. Simply pickup a set of keys. You're not looking at the proof that all people are inherently evil. If we were inherently good, no one would need keys for anything, because no one would have the desire to take what isn't there's. But since the opposite is true, we lock our doors. Our houses, cars, all our valuables. We lock everything up so people won't steal it. Yes, a determined individual will still find a way to take it, but the one's who are mostly honest will leave it alone. If it wasn't locked though, there'd be nothing stopping them, so they'd take it.

My point is that while Cho probably had some help and discipline when he was younger, he was obviously missing some self discipline while he was at college. There's really no one else to blame here except Cho. The only problem is, he took himself out with the rest of them, so there's nothing that can be done except to mourn with all those involved.