I'll make this quick, but Centuri0n posted some thoughts on the Virginia Tech massacre that also dealt with the concept of God's providence. He made what I thought were two crucial points:
1) "I am just like Cho Seung-Hui -- not that I am unlike him and he's the one who did something God hates. I am like him. If I am honest, I can see in my own life the moments when I could have gone one step farther than I did in some sinful act and stepped into a life which would have meant that I was the one who would have killed 30 people who didn't even get a chance to be grown-ups yet...He could have been me: I am a sinner, and I am the cause of sin."
2) "God allows these things in order that a greater redemptive purpose can be manifest in Creation. So that nobody gets their nose out of joint more than I mean to put it, this purpose is God's purpose for God's own end and intention -- but it saves men."
He at the same time points to the depravity of man, but also to God's sovereignty even over such a brutal, mind-numbing event. This ties back to the discussion we had a month or two ago about predestination, because ultimately I may not understand God's purpose in allowing sin, but I know that he is sovereign over it. It's a mind-boggling thought, but it's comforting in a time like this.
I'd encourage you to read the whole article, he makes some very helpful points.