That's probably a reason why I see the alignment system as a general guideline more than anything hard and fast. In my RL game my (14yo) son plays an evil cleric. He's obsessed with the idea of alignment but terrible at actually being evil. He'll help the old man who's been infected with ghoul fever rather than killing him or waiting for him to die and raise up so he can control the resulting ghoul.

Of course nothing says that an evil character cannot help others, form friendships, be nice to people and pay for his own beer but being nice to people just doesn't seem evil. I've tried the Robin Hood analogy to explain alignment to him but really alignment sometimes seems less that useful.

Robin Hood analogy of alignment
Deep in Sherwood forest live the band of assassins, brigands and murderers ruled over by the master assassin, Robin Hood. Robin regularly attacks peaceful merchants, nobles and travellers, robs them and kills them where they stand if they show any signs of resistance. The Sheriff of Nottingham has sent his best men after this fiend, declared the fact that he is an outlaw across the land, offered rewards for his capture, everything he can think of short of burning villages and torturing people for information. These he would never do for the Sheriff is a good and noble soul.

Once or twice the Sheriff and his paladin knight, Guy of Gisborne, have come close to capturing the brigand or caught one of his henchmen but always he breaks into the castle, murders the guards and escapes. The sheriff insists on a proper trial and that usually gives time for a rescue.

Somehow the castle has a leak as well. Gisborne suspects the fair Maid Marion of being responsible but has no proof. He is right, Marion is wicked and cruel, an assassin herself, a poisoner and vicious killer who sells secrets to Robin and her men.

The worst thing is the peasants. Robin bribes them with food and gold, claiming to be good and just and returning only a portion of the taxes stolen from Nottingham and forcing the sheriff to put up these same taxes. This money would go to better roads, better law enforcement, education and medicine for the poor, and to attract new business into the area to reduce the tax burden on everyone. If only they could find and kill Robin and his band of murderous men.

It is supposed to show how alignment is easy to fool. You can take any action you like, more or less, and justify it for any alignment. The classic example is killing a prisoner. Lawful good might call it a lawful execution, regretful but necessary. Chaotic evil might kill him because he wanted to hear him squeal like a pig. I'm sure you can think of other reasoning that fits each alignment.