if they are not playing a chaotic evil homocidal sociopath...they are likely not a chaotic evil homocidal sociopath.
the problem i see with your view on nuetrality is that you are trying to hold it to the same standards as good and evil, while by definition you clearly cannot. Say a goodie seeks vengeance and kills an undead because his family was massacred by it when he was a child. RPwise, it is a selfish reason. He killed it not to bring light to the world, but to strike back at it for the misdeed. by your definition of alignments, he would have committed an evil (at worst) or nuetral (at best) act.
PKing nuetral by no means must lead to evil. By no means does it need to lead anywhere but nuetral. Good and evil are defined more easily as a moral outlook rather than deeds or misdeeds. nuetrals are a rather loose term. PKwise, i see nuetrals as the ones who:
-
Like to fight. Perhaps they favor combat instead of worrying themsleves with devotion to anything other than the blade. Here is where I see nuetral warmasters. Fighting to the stun can, for instance, be shown as arrogance. Perhaps you fought to the stun in order to humiliate them, whether for their benefit (they want revenge and train harder) or your fun (the world sees you winning more fights, you gain more fame). Fighting to the death can be a means of eliminating your competition before they get too powerful, thus keeping yourself in the focus. Just an example.
-
Fighting for a cause. Not all causes in this game are good or evil. This is where I see Watchers. In the same way that the goodies fight to spread the light, the Watchers fight for Gaia. They fight civilization, who desecrates Gaia. They fight Avatars and Undead, as they are mutations, and therefor unnatural. They fight Demons, as they do not belong to this realm. In essence, they fight to destroy anything that is not a part of or harms nature. This lends itself to neither side in the war between the Gods of Light and the Gods of Death.
-
Heat of the moment. You can have tendencies without devoting yourself to them. In my example of the brawler, you can get carried away, perhaps under the influence, without intentional malice. In fact, i even see goods killing a nuetral or other good as something that, if RPed correctly, does not neccesarily have to mean that they are abusing their alignment. In fact, I believe it could be a fun RP angle. Perhaps a paladin does it. Stricken with guilt, the Paladin seeks every chance at repentence he/she can find. He/she is still fighting evil for a greater purpose. He/she is still giving of his/herself to aid the innocent. If anything, perhaps moreso in his/her desoeration for repentance than other goods who have not committed the act, yet are still viewed as "more good."
the list can go on and on with specific examples, but to be frank, I don't have the time to post them all.