Good and evil in D&D is not subjective, it is objective. Good is being merciful, forgiving, and kind. Evil is being selfish, harmful, and malicious. Even if a character does not think he is doing evil it is not the thought, but it is the action that counts.

Also, paladins can hardly get away with doing neutral acts. For example, if a paladin were to pass by someone innocent in trouble he would be violating his code. Whereas a neutral alignment would be able to do this. You serve a deity as a paladins, if you don't uphold the deities wishes you lose your powers. Paladins aren't meant to be played by temperamentally. You're given responsibility with that class, and if you ruin it, you lose your privileges too.

Torture's full purpose is to inflict pain and suffering on an individual, and is therefore an evil act. I don't care about bob the rapist. As a paladin it is your duty to uphold the law and your deity's values. If that means you don't kill people unlawfully or without just reason then you don't do it period. There are no exceptions.

A kensai is a good example of another class that requires a huge commitment (to a more extreme level). If you violate your code for any reason you lose your powers and have to atone. It doesn't matter whether it was for the greater good or not, you violated your code.