Kamelguru, I think you forget something important here. Let's say a priest of Abadar decides he prefers channeling Negative Energy rather than Positive Energy in order to smite the ennemies of his church rather than heal the faithful (or the paying customers).
Channeling negative energy is evil. Therefore at level 2, you're going to smite that priest of one of the most widespread, accepted and important religions of Golarion ? Well done, you're now a criminal and good luck justifying yourself. "He's evil" ? Sure he is. Did he do anything wrong ? no. Did he abide by the law ? Yes. Did he hurt anyone he shouldn't have ? No. Did you ?
Yes.
Now, let's take it one step further. An evil illusionist uses a simple spell to give a commoner an Evil aura. With your Detect Evil ability, you detect the aura. And smite on sight. Congrats, you just murdered an innocent.
And there's no justification for what you've done. But it doesn't matter anymore to you, because you're not a paladin any longer. An Evil aura is a sign that something is amiss and worth investigating. "Smite on sight" is never, ever acceptable.