Blaming victims is an incredibly popular pasttime, and it, unfortunately, contributes to a great deal of harm. When we blame victims, we make life harder for them and tend to oppose changes that would help them or prevent more victimization. So, why is it so common to blame victims?
Well, there are several factors that encourage people to do so. First, the world is scary. Lots of bad things happen in the world. And worst of all, you can be pretty certain that eventually you and everyone you love will die. People don't like to think about that. When people see someone who has undergone a terrible hardship - a horrible disease, been the victim of violence, or had a major accident, for example - they want to be reassured that it won't happen to them or anyone they care about. Alas, there are no guarantees in this world. Sure enough, you might end up in one of those situations yourself, if you aren't already. But if people can find something about the victim that is different from themselves, then they can pretend that it won't happen to them because of that difference. So, people think to themselves, oh this person was assaulted because she was walking around alone, but if I don't walk around alone then I'll be safe. This isn't necessarily true, and it certainly doesn't make the victim of the assault to blame, but it does make people feel better. Then it's just a short hop to this person should have known better to do that and shouldhave let her whole life be ruled by fear on the off chance that something bad might have happened if she didn't limit her actions very carefully.
So, we blame victims to make them something different from ourselves. And this means that we tend to dehumanize victims. If the victim isn't like us, then the victim isn't as important as "us" - the victim is one of "them". Dehumanization is at the heart of most cruelty. It is far easier to abuse or kill someone if you don't view them as human. In wars, you regularly see people dehumanizing the enemy, and often even the civilians. In various psychology studies, people have been shown to be more willing to shock a person (even unto death) if the person is in another room than if they have to see the person (the shocks were faked, but the participants didn't know that). The more someone has to interact with a victim and directly harm them, then the easier it is to harm them.
The problem we face is that we can do a lot of harm from a distance, both in wars and with legislation. What we need to do is recognize that someone is still a person if they are poor, ill, of a different race, or so forth. And we need to have the courage to accept that bad things might happen, but if we work as a society to help the people they happen to, then we will all be safer fromt hem. We cannot ever get a guarantee that bad things won't happen to us or our loved ones, but we can create a society that will help us out if they do. We can trade our unhelpful ways of hiding from our fears for true ways of making the world better.