If someone does no bad, then he can't do good.
The logic in that is like saying, if water can't burn a house down then it can't douse a fire. We as humans naturally do bad, it is in our nature, but of course we naturally do good as well. Christianity does not assume to make someone a "good" person, nor does it proclaim to give you amnesia from all your sinful thoughts/actions. Instead it gives you tools to deal with it.
If you see good people, you find bad sides about them. But they may hide them very well. If you find very good persons, you find very bad aspects of their personality. If you don't find them, you haven't lived with the person long enough or don't know them good enough.
Whatever lies hidden in our own personal private thoughts definately goes in between the good and bad. But humans have something called choice, we are not bound to an instinct like most other animals on this earth. We can harvest negative thoughts but make a choice not to act on them, or at least reduce the intensity that it invades our private mind or leaks out into our physical action.
So when you say now:
So that's pretty much why i'm not really into striving for being good *g* - of course i do a bit, but i allow myself to be bad sometimes; and that is actually a very strong experience - to know you can be bad, have a bad mood, and the others accept it as part of you and don't refuse you for what you are.
This does not give us permission to feel negatively at all. Although we do bad and can't help it it doesn't give us the right to do it. Nor does it give us the right to act bad with no remorse because of a learning experience you gain from it. You gain a much more important lesson from life if you learn to control what you naturally want to do instead of letting it uncontrollably run your life. You might not necessarily stop yourself completely from doing it but you definately will change how it affects your life.
You see if you have say a bad mood and get impatient with people you simply do not know what you are doing! What gives us the right to think that people owe us anything? Why does the world owe you anything? You might feel anger for a moment but you learn not to let this come into physical existance with words or actions. Mastering ourselves is a life long journey, it is weak to simply say to others this is my nature deal with it or you don't like me. It is what makes a child molesterer a predator, they know what they really want, they enjoy it so they'll do it without remorse and think everyone else has a problem. That might seem like an extreme example but it reflects the negative urges in our own life. You simply might not think that having an impatient miind is as serious as having unlawful sexual desires and you wouldn't if you do not have a God to answer to. As Christians we cannot really weight one sin against the other, all sin breaks our relationship from God, that is all we need to know.
The christian ideal doesn't really make sense in that way...
Chrisitians proclaim that we all sin and break our connection with God every awake moment. It definately does not say that if you are a Chrisitan you will glow with magical light and be a pure and good person.
A person being Good or Bad is a simplistic model, in reality, as you have also noticed, we are Good and Bad simultaneously, a ying and yang. There is a constant descision making for us to make that choice between right and wrong. Christianity most importantly gives us a connection to God and Christ which guides us through our Life and Death. With respect for a God you gain a greater respect for yourself and all people in this world.
If you respect only your own brains and logic then you serve an imperfect model. Those without God start saying, well I don't need God because everything I do in my life I have thought with my own head and done with my own actions without a God helping me. This is exactly what a Chrisitan does as well except; We do things in our life with our own thoughts and our own actions AND we are constantly asking a greater presence to guide us and give us the right words to say, right actions to do, right people to meet. We do not stand in a stupor waiting for a Voice to say GO FORTH, we feel God as we are resting and moving. We feel god when we plan, we praise god for giving us insight. We might naturally think these things without praising God but the fact that we acknowledge that our inspiration comes from God, this gives us greater appreciation and care of what words/actions that come from us.
A lot of discussion about Christianity when you deal with believers and non-believers should remain on the "Respect for a God". That is where it all starts. If you have no respect for a God then describing the other functions of Christianity to you becomes somewhat useless. It is like studying an overly difficult piano piece and ignoring the easiest part which you still find hard, and go into more confusing parts instead.
Respect for a God should not rest on I need Proof of God. Rather it should rest on how we deal with our thoughts and actions. We have to make a choice whether we do things in the name of humanity or God. You will find those who worship the world definately start to despair when death hits them. No atheist feels comfortable about death, they either feel apathetic to it or fear it. I would like to meet an atheist who says Death is something to look forward to, an athiest worships the material world for they have no god, so they want to exist in this material world as long as they can. Some might not want to and yearn to return to the abyss of non-existance from where they came. Sounds too dark and gloomy for me.