Evil is simply the willingness to watch or cause others to suffer for your own gain.
Isn'r suffering a big part of Christian beliefs? Especially older ones? If memory serves even Mother Theressa was a believer of suffering bringing people closer to God.
Anyways, back to the question:
Evil is a relative word, with it at one end and "Good" at the other. Like Big and small they have meaning in relation to other things, but not on there own. Is a Elephant Big? In relation to a Insect or a planet?
It is also culturally defined. Is Christianity "Evil"? In 21st century North America? The Middle East? How about 1st Century Rome?
Was Paganism? Pre-Christianity? Dark Ages?
Even nowadays we have fuzzy issues. How about Capital punishment? Eating Meat? Scientology? Fundamentalist religious beliefs (Islam or Christianity)? Preemptive attacks on countries that are unable to effectively defend themselves (Iraq)? Communism? Capitalism?
But lets take one, Human sacrifice, something we all will agree is pretty bad, and I'll see if I can defend it
Whether it is evil or not would at least partially depend on what you believe the reprecutions of it to be would it not? So if for example, you believed that a sacrificed person would be greatly rewarded by the Gods, as well as the village recieving a better harvest or some other thing that is good and saves lives. Is it evil? Our beliefs tell us that sacrificing a person won't improve the harvest or bring rain or a appease any Gods we believe in. But, if your beliefs say that it will, does that change things?
Let's say you see a man who is starving, and you give him some food, warm clothes and a place to sleep. This act is "good" is it not? Now suppose he dies of a unknown allergic reaction from something you gave him, does that make you evil all of a sudden?
No, you're belief was that these things would help him, possibly save his life. You where acting out of generosity and kindness, trying to do some good, and through no fault of your own the man died due to a condition no one knew about.
Now lets take another case, you are a doctor and a patient has cancer. If you do nothing you believe the patient will die within 6 months. However you can save the patient by amputating his arm. You do so, and the patient lives out the rest of there life healthy, but armless. This is a "good" act. Now 3 months later someone else cures that form of cancer, had you not sacrificed the arm the patient would have been able to have been cured, and lived out the rest of his life healthy with both arms. Was making the sacrifice evil?
No, you acted on the best information you had to do what you believed to be best for the patient as a whole. The fact that it was cured could not have been known to you and you acted with good intentions.
Now how about a military situation, where the entire crew is going to die if a problem is not fixed. One person could fix the problem, but doing so will cost them there life. You order someone to go, saving the lives of everyone else through sacrificing a human life. Is it evil? How about if rescuers arrive minutes later and could have solved the problem without costing a life, but you did not know that at the time?
Now you are the leaders of a village, you and the entire village believes the Gods have been angered and require a sacrifice. You ask for volunteers, and someone steps up to sacrifice there life for the good of the village. You and they both believe this as strongly as you believe feeding a hungry man will help him, or amputating a cancerous arm will help a patient, or a Christian priest believes in God. Is it wrong to make the sacrifice?
Where does the evil line get drawn?