Ignorance is something that people don't often realize they are. They can be taught, they can be educated. If you truly believe something to be right, you are justifying to yourself, and often, that is all that matters. That doesn't mean I condone someone killing someone because the murderer thought it "just". I'm just saying that if you do not realize it is wrong, steps can be taken to prevent it from happening again. It's like when a toddler hits you because they don't know any better. They learn from your example of not hitting, and then, they know not to do it anymore. They have learned from their mistakes, even though they didn't realize they were making a mistake in the first place.
Knowing something is wrong and doing it anyways is something that is very, very cruel. You are most likely doing it for your own personal gain, and knowingly doing something that is considered universally wrong is not good. For example, if you were stealing a candy bar from a corner store. Maybe you conscience is whispering to you, but you slap it away, and think "No, I don't care. I'm doing it anyways, because I want to." Obviously, there are more severe cases, but it can be as simple as, "I don't want to do the right thing because I am too lazy, cruel, tired, bored, etc.
To finish off, here is a hypothetical situation, one with someone knowingly doing something wrong, and someone doing something wrong without knowing it wrong, or believing it to be right:
Sally Jane is walking to school one day. Her friend catches up with her, and starts talking. Sally Jane's friend is very upset about something that Sally thinks is silly, and she tells her friend so, not because she wants to hurt her, but because she is trying to use "tough love" to snap her friend out of her slump. However, Sally's friend bursts into tears after Sally has finished telling her how ridiculous she is being. -End example 1-
Sally Jane is walking to school one day. Her friend catches up with her and they start talking. Sally Jane's friend is upset, and Sally is tired of hearing her friend talk about it. Deciding to end this conversation now, Sally Jane cuts her friend off with no warning and changes the subject. Her friend is extremly dejected and upset, and Sally knows it, but thinks she should suck it up, as her situation is stupid anyways. -end example 2-
The difference between those paragraphs is that in the first one, Sally Jane wasn't intentionally trying to hurt her friend's feelings, but did anyways. In the second, she was intentionally trying to hurt her friend's feelings for her own personal gain. In my opinion, Sally Jane from the first paragraph is more justified just because she was trying to do the right thing, but it ended up being the "wrong" thing.