Why are some non-Christians nicer people than many professing Christians?
It's easy to be nice when things are going right in your life. While working retail, I've witnessed people from all kinds of backgrounds change from smiling, happy people to ranting, raving lunatics in a matter of seconds because they were told something wasn't in their size, something couldn't be returned, or some other thing. Christians are people with good days and bad days. Since you cannot know what is going on in another person's life, it's unfair to judge a whole group based on the attitudes of a few.
Besides, how do you know that the person professing to be a Christian really is one? Many people think they are Christians because they go to church, believe in God, etc. But have they truly made Jesus the Lord of their lives?
If they have, great! Still, it takes time to change habits and behaviors that have always been there. It is unfortunate that many Christians, even after receiving salvation, still don't understand that they are a NEW CREATION, with a NEW NATURE, and so they fall back on their old nature. They've either never read or understood that their "attitudes and thoughts must constantly be changing for the better" (Eph. 4:24) or that, "quarreling, harsh words, and dislike of others should have no place" (Eph. 4:31).
I am sorry you have been hurt by Christians or by a bad church experience, but please, don't let what others do keep you from giving God a chance. People make bad choices when driving, too; they treat it like a game instead of the enormous responsibility it is. Yet, you didn't let it stop you from driving, right? So, why let others decide for you whether you follow Christ or not? Give Jesus a chance. He's worth every bit of your time, heart, and energy.