Evil is a matter of perspective, what some see as good others will see as evil and what some see as evil others will see as good. Almost nobody, with the exception of the ignorant and/or misled, actually try to be what they themselves see as evil. I define evil as anything that disagrees with the moral beliefs of an individual, making it a personal problem rather than a universal truth.
By my definition you can not ultimately protect yourself from evil. Everybody's beliefs are different and unless you choose Hitler's path to make everyone the same (which would cause many to see you as evil) everyone will continue to have different moral beliefs.
That said, you can shield yourself from negativity and shield yourself from harm through simple energy manipulation practices. You can also ask that your spirit guide(s) help to keep negative energy and harmful energy away from you. These are things you should, and hopefully will, learn to do before attempting to astral project.
Temptation, on the topic of negativity, I might disagree. I say "might" because it's possible I misunderstood your post. Anyways, if a person learns to let go of everything that could cause negativity and live worry free and stress free life, that person will have no negativity in them. They will encounter it possibly often, thus the practice of letting it go, but encountering and dwelling on are two different things. Buddhism is a philosophy that focuses on this matter.
As for your question about who is to say what is and isn't evil, everyone. Everyone has their own moral compass and so everyone has their own definition of evil. Usually these definitions are very similar: Kicking an elderly lady in her shins as she attempts to cross the street would be an act of "evil" to most people.