The second through fourth reasons seem to beg the question rather than answer it. While not all immoral actions may be evil, all evil actions are necessarily immoral. The second reason is focal point here, because the last two can be subsumed under it. So when you say that an action is immoral if you know it to be evil - since 'evil' is a particular type of immorality - it's like saying you know an action to be immoral if you know it to be immoral. This doesn't answer why a given action is immoral, though.
In regard to your first reason, it seems to be another case of begging the question. My conception of integrity is that it means to act in accordance with your moral values; that your action corresponds to what you believe to be right action. To go against - to violate - your integrity would mean to perform an action that, by your own standards, you believe to be immoral. Thus the nature of integrity already assumes a moral standard of right and wrong. The first reason seems to be saying, then, that you know an action to be immoral if acting on it would be immoral by your own standards (it would be going against your integrity). But this, again, doesn't answer why we hold such action to be immoral in the first place.
I was arguing how one knows that an action is immoral, not why they are immoral. And I believe Evil and immoral are the same thing.
I'm not sure I see a difference between how one knows an action to be immoral and why an action is immoral. The former seems to imply a knowledge of the latter, in that if you can identify an action as immoral seems to mean you know what makes an action immoral in the first place and how the given action meets the criteria.
Although if you believe evil is the same as immoral, then wouldn't that mean that your second reason to the question of what makes an action immoral is that you know it to be immoral (since evil and immoral are synomynous from this view)? An action is immoral if you know it to be immoral? This would seem to still be begging the question, though.
I'd explain my claim that "While not all immoral actions are evil, all evil actions are necessarily immoral" by mentioning that the difference is one of degree in regard to an action. This requires an example of a moral principle, so for the sake of this I'll simply assert one to explain my view: that the initation of force against another person is immoral. Now consider the first case in which Person A slaps Person B and then walks away (they were arguing or what have you). I'd consider that to be immoral by light of the above principle, but not evil. But imagine a second case in which Person A not only slapped Person B, but then proceeded to assault and murder them. It's then that the label of evil, by light of the above principle, seems fitting. What separates an immoral action from an evil one is a matter of degree.
-- Updated Sat Feb 04, 2012 4:58 pm to add the following --
Fanman wrote:I think that the answer to this question is priori, in that we all "know" what makes an action immoral. However, articulating the answer in an objective philosophical manner is not so easy. Essentially, I would say that an immoral action is any action that is commited dishonestly by a person, which causes harm to another person.
In regard to the last sentence, what if a person honestly commits an action that will bring harm to another? The last line seems to exclude this possibility. You could imagine a person pretending the drink he offers to his guest is safe (when he has, in fact, poisoned it) and thus being immoral. But an individual could also state that his drink is poisoned and try to force the person to drink it; there would be no dishonesty in this and, if successful, would still cause harm.