You have to accept people the way they are. You're allowed to judge people on what they do.
I believe niceness is a set trait. One person is naturally more or less inclined to do good than another.
Niceness in people is spread out according to the normal curve.
I have to admit that I too much, too often, have wanted to judge people as nice or not nice; have wanted to put them in boxes instead of placing them somewhere on that curve.
I probably did that in times of uncertainty about myself, to compensate for that uncertainty I tried to create certainty in my surroundings.
But you have to accept people the way they are. That's what makes life easier for you.
You don't have to accept everything people do however. Despite the fact that people are naturally more or less nice, (almost) everyone knows what doing good is. You may therefor expect them to indeed do good, for they expect others to do the same for them as well.
The person that does good despite him being naturally less nice, is more applaudable than the person that does as much good while being naturally nicer.