You don't have to be mean to not let people walk all over you.
Being nice does not mean you can't say no, draw lines in the sand when required, and not over extend yourself.
Personally I am the type of person who is a welcome mat personality. I try so hard some times to draw the line, some times I do, but...
Mostly what it comes down to is I'm a weak and timid person. I have no one but myself to blame for letting people take advantage of me. I do try, but, I usually just default to letting people step on me. I'm used to that, I'm used to that kind of pain. It's much easier. If it were easier to stand up to people and not let them take advantage of me, I would surely do that, but, for me it's not.
I don't get why people have to be so bitter about it. If you are a weak person, just accept it and enjoy all the crazy horrible missadventures and nightmares your weak character will take you on. That's what I do, haven't been able to come up with any other way.
A few times I have pushed through the brick wall and really surprised some people. A lot of the time if you show a little 'foot down', people who seem tough and non-negotiable will really crumble like a house of cards, and you'll wonder why you ever thought they were such an obstacle in the first place.
Not always, though. Some times it's good to let people walk all over you. Like if you are being robbed at gun point.
I say don't be bitter. Either accept what is, or make the decision to continually work on improving your reactions to people. Either way you and the choices you make are your responsibility. Serial killers can be nice. Don't worry about being nice. Just worry about being a person of good character and making choices that are in the best interest of everyone involved in them, as best you can.