Good, in the eyes of many philosophers, is a desire to aid others at the expense of oneself.
So most people aren't good. They don't want to hurt anyone if they can avoid it, but they don't put much effort into helping people, and when they do it is usually out of a sense of obligation or reciprocation. 'I help you move into your new apartment, you'll help me some day.'
A good person would look for ways to help people who don't ask him. He'd offer his money to the poor, rather than guiltily handing some over when asked. He'd volunteer to give books to underprivileged children, or offer rides to people walking in the rain, or fly to another country and risk his life to provide medical aid.
An evil person, on the other hand, desires to harm other people, particularly if he does it alone, rather than at the urging of society. If someone gets off on torturing people, he's evil. If someone tortures because he's been told to by his superiors, he's not necessarily evil. If you spread gossip and try to get people in trouble because you like it, you're evil. If you're just bad at keeping secrets when people ask you questions, you're not.
The weird thing here, though, is that hurting a group your society doesn't consider 'people' doesn't make you evil. Shooting your enemy in a war is fine. So is terrorizing people you think are threatening your way of life. It's misguided, but it's what your society considers right.
Really, a good bellwether would be how you'd act in a completely foreign environment, without someone to approve or force your actions, and without societal connections to any local group. Assume you have enough money to take care of yourself. You come across someone who looks sad or sick. Do you stop to offer help? You pass a car that has run out of gas. Do you offer a ride to a gas station? You hear shouts down an alley. Do you go see if someone is being attacked?
Now, ask those same questions, but you're in a hurry to get some place. How much are you willing to inconvenience yourself to help others?
Dexter? I have no bleeping idea. He acts out of the bounds of society, and disposes of dangerous people, but he enjoys hurting them. He could become a cop or a detective and track down bad guys as part of society, but he doesn't, because he wouldn't be able to fulfill his desires.
The best I can say is that he's not Lawful, because he's not working within the bounds of society, and he's not Chaotic because he's not working to undo society. I'd say he's Neutral on the law-chaos axis.
He's not Good, because the only magnanimous thing he does is kill people. His life, in general, is not motivated by a desire to help others. But is he Neutral, because he's only killing people who don't count as people, or Evil, because regardless of who he's doing it to, he enjoys causing harm?
I'd say Evil. Neutral Evil.