In the beginning I'm going to tell you a big secret: Humans are not the only creatures able to commit crimes (chimpanzees commit murders and even some ants are able to enslave or kill other ants for nether motives), but they might be the only creatures who can be evil. Now you might say that 'committing crimes' is the same as 'being evil', but it isn't - even though the principle of 'crime' is a human one, too.
Fact is, to be 'evil' you have to know the concept. Being evil is something you do by choice. And animals, at least as far as we know, don't have a definition of 'evil' and thus aren't able to do anything evil. They can do bad things, but it's their nature, nothing more. We don't expect them to change their ways, we don't expect an ant to rise from the heap and tell all others that slavery is wrong. We don't expect chimpanzees to form a CSI-unit to find out who of them did it. We judge their acts and see them as criminal or even evil, but to them they're just what they do - nothing more, nothing less.
But now on to the main topic: Evil.
I personally don't think there's a clean line between 'Good' and 'Evil' as some religions make you think. There isn't the precise moment where 'Good' turns into 'Evil' or the other way around. It's a slow process and in the end it's impossible to say where something went wrong.
Then you have to divide between 'willingly being evil' and 'doing things for the greater good'. Most humans don't even make it to 'willingly being evil' (even those absolutely characterized as 'Evil' in the end - like Hitler). They stay in the large realm of 'doing bad things for the greater good', a reason that can be used to excuse almost every evil thing humans can do to each other.
You see, if you believe in 'doing things (even very, very bad ones) for the greater good', you don't see yourself as 'evil'. I highly doubt Hitler and his fellows ever said "let's go kill some people just because we can" (they did it, but they never said it out loud). They thought they were doing this for the good of the German people. Well, a lot of good this has done us... But if you look back at history (even before the 20th century), you'll find that people 'doing it for the greater good' did more lasting damage than everyone else. They go around wiping out a great mass of people and all the while think they're doing something good. Think 'Inquisition', for example, or think 'Witch Hunts'. Think 'Crusades' or think 'Jihad' (which, actually, is pretty much the same, only seen from two different sides). Half of the time people go around murdering each other, they claim "it's for our god, really". If there is a god (and some days I'm not really sure about that), She is sitting up there and patiently developing new ways to torture all those bigots who are killing off others in Her name. The other half is more honest and says "I'm doing this for myself".
'Willingly being evil' is something a lot of Satanists seem to attempt. They are not doing things 'because something good will come from it', but simply because they think they are supposed to be evil and do very bad things to other people. They live by the principle that the stronger one is entitled to do bad things to all those who can't stop it from happening. If whole countries were officially working that way, George W. Bush would not have had to claim "Iraq has weapons of mass destruction". Then he would just have had to say "We're stronger and we want the oil and we hate everyone who's not white, Christian and from European descent, so we'll wage war on Iraq and kill whomever walks into our path". But you can't say that as a president of one of the most powerful countries in the world. It would be honest, but not politically correct. Saying you're doing this for the greater good (seen something you might recognize?) is always more acceptable - and maybe, just maybe, they're even believing their own lies.
But most of the people are neither willingly evil nor doing things for the greater good. They are evil in a very mediocre way. I don't even mean people committing crime, that's mainly motivated by egoism and is thus something that can be explained. But there's bad things people do without any apparent reason - like mobbing someone who's no threat to the mobber's own career. That's something I can't really understand either. I can understand undermining the standing of someone who could be a thread to my own career (even though I wouldn't do it myself). But if someone is no threat whatsoever to my career or personal life plan, there's no reason to invest time and energy in making that person's life miserable.
Unfortunately not all people think like this. There are some who actually wallow in this, in seeing someone suffer for no real reason. ('Real reason' for me in this case means 'a reason I can understand, even if I can't agree with it'.) Sometimes I even think they cause this much misery because they actually gain something from it. Maybe they do, at least some satisfaction.
It would be up to society to make clear this is no acceptable behaviour, but many companies (and other places where a lot of people come together and social positions are important, like school) rather support it. During the last couple of years everything in life has, to some people at least, become a 'war' (which is why a lot more people than before have been reading Sun Tzu's "The Art of War"). Holding you job? A war. Winning a tennis match against a friend? A war. Finding a woman/man to spent the rest of your life with? A war. (Strangely enough, Sun Tzu himself was absolutely clear about the fact that his thirteen chapters on warfare were not to be used in times of peace.) The question I always wanted to ask those people is: "Are you happy to lead a life in which everything is a war? Does it give you satisfaction?"
For those people, making others miserable seems to be essential to leading a 'good' life. They always feel assured of their position and their way of life after they've managed to bring somebody else down, I guess. And as long as everyone says we're living in a world where only 'the strongest will survive', they will continue to act like that.
So 'Evil' from my point of view has only little to do with the devil and his demons. Humans are far better at being evil, just by acting 'human' in the worst way that word can be defined. I guess I will never fully understand what people gain from it, but I think the society does have the responsibility to make sure this dark side of our nature is kept in check better than it currently is.