Does not accounting for bad choices make them bad people?

Or just ignorant? 

I've always believed people don't do bad things on purpose. That no one would like to cause another pain or hurt on purpose. Sometimes it's understandable, when their happiness is at the expense of yours. I guess that kinda I sorta understand. It's always ''do what makes you happy'' and not the other way round.
But I do wonder, what about them who just..don't have reasons? Why do they do the things they do? Even to their own close friends?

Like in high school. I had this friend who blogged about some really private 'activities'. Sure, it wasn't really a good idea to blog, seeing as it's a public blog. Our friend, another guy..came across it. He read it, printed it out, and photostated tons of copies. He spread it around school and it went viral in Cheras. Apparently people still talk about it.
Why did he choose to do that? What did he gain? Does he find some kind of satisfaction or some sort of happiness in watching her being publicly humiliated? Sure, she did the crime but why must he come into the picture? Why couldn't he have kept quiet?

In my college too. There was some scandal of this senior couple going at it and it was recorded. A close friend of theirs got the video and she spread it. The video was bluetooth to almost everybody, even during lectures in the lecture hall. A friend of mine told me this (yeahh everyone was talking about it but it didn't reach my ears till MONTHS later =.='') and well, I see this couple and that girl everyday. I wonder, what is it like for them? And how can she walk like that? With full confidence like the world's her stage, talking to everyone, smiling. If I did what she did, I don't think I can live with that guilt. But then again, I wouldn't.

And another case. How can a best friend of 8 years use her to get to some guy she like. To go behind her back, lie about it. And her ''best friend'' had to find it out on facebook.I guess the 8years of friendship means nothing when compared to some good looking guy on facebook.

How do these people live with themselves?
Hurting others.
Ruining friendship.
And walking away, like there was nothing there to begin with.
Like no damage was done.

I don't get it.
I don't understand.
Where's your heart?