This is something I've spent a lot of time thinking about, though often it simply goes around and around in my mind, and there are many situations I think of where it's hard to find a satisfactory answer. I like a lot of the things that have been said here. As Merv said, I think to some extent we are all responsible for our actions--if something terrible happens, such as a crime is committed, you have to take into account all the parties involved, including the perpetrator, the witnesses, and the victim. When it comes down to it, I think you have to look at it on a situation by situation basis--the victim could have some responsibility, [such as, perhaps if the victim was murdered, he/she might have been blackmailing the murderer, in which case there was some provocation], or he/she could be entirely blameless.
I also agree with levina that, even if we can't help how we feel about something, we do have control over our actions.
As for the question of where to assign responsibility if someone does something that results in something they did not intend--that's a hard one, but in my mind, I would separate the action in question into two categories. And that's whether the person ought to have known if behavior in question was likely to have a negative result or not, or whether the action was random, and no one could have expected it to have the result it had. You might say, whether the action is one we should have known to be wrong or not. In the case of bullying and the victim later committing suicide--it's true that the bullies might not have expected the victim to commit suicide, and that wasn't their intention, but at the same time, some reflection would have told them that it would hurt the person, make them miserable--and when people are miserable, they are far more likely to do something extreme, whether that's commit suicide, or do something extreme to hurt the bullies. In other words, the act was one associated with a negative outcome, even if the more extreme reactions aren't usually expected to happen, since they are rarer. Or, someone might be drinking or taking drugs--that would be another high-risk activity that could result in negative results for either themselves or others.
On the other hand, looking at what you might call 'the butterfly effect.' (That is, they say a butterfly flapping it's wings in some far off country can potentially cause a hurricane here. I'm not sure if that's true, but that's what they say.) The idea is, of course, some small, seemingly unrelated event results in something huge. Say, you normally carrots with your lunch, but one day you decide instead to get green beans. But it turns out the green beans were on recall because they were contaminated with something that ends up making you sick. There's no way you could have possibly predicted that, so that type of thing wouldn't be your responsibility in any way. Or take a look back at the late 1920s, when the stock market crashed--it's true that a lot of people who lost were investing way too much, and taking too many risks, but even the people who had been careful and were doing everything right lost, too. They couldn't be held responsible for that.
Anyway...ultimately I guess I think it can be complicated. Whose responsibility something is isn't always straight forward, and many times it can be a mix of different factors, some where someone did something wrong, some that were just bad luck. I think many times the important thing is to just focus on problem-solving, if possible, and healing rather than trying to figure out who's to blame.
(I know, this is a convoluted response. I shouldn't be doing this at this time of day. xD I think I've lost what I was trying to say...)
“After all, absolutely no one can help but suspect a criminal, liar, and manipulator of committing crimes, lying, and manipulating. And of course, no one is more aware of that simple fact than Artemis Fowl.”
Opal sets into motion her most diabolical scheme yet, to frame Artemis and turn his closest friends against him. Only this time she has a new calculating partner who knows Artemis better than he knows himself. [An Artemis Fowl fanfiction, set after The Atlantic Complex.]
...Shameless self-advertising, guys! C;
(And if you're really bored: AF fanart. ;J)