We must call atheist's to be consistent with there own proclaimed view. Obviously the meaning of atheist is the concept that there is no God. Frankly it's the "belief" that there is no God, even though they hate that word to be used of their view.
Frequently I have found when speaking to, reading the writings of, or listening to an atheist speak, the most common objections against the idea of GOD is that there is so much evil in the world. Therefore, by their conclusion, there is no God. After all if there really were an omnipotent, omniscient, loving being, then that being would not allow such evil. This is from my experience the most common objection (to the God whom we as Christians worship) that atheist usually give. I'm not saying that they don't object on the basis of other things also. Many atheist claim that science and the Bible conflict in an irresolvable manner. (Of course many scientists find that the Bible and good science always agree.) Regardless of what other objections there are against the idea of GOD, evil is the most common problem in the minds of most who reject the idea of a supreme being.
The ironic thing about this ever common objection to God, is the fact that the very objection defeats itself. To say "There can be no God because there is so much evil" is a ridiculous statement. For if you throw out the idea of God, then by default, you also toss out any real definition of evil. What is evil? Who decides what is evil? Is not evil the opposite of good. Who decides what is good? Do you see the problem?
The Christian appeals to God as the ultimate standard of good. Good is whatever God calls good and evil whatever He calls evil. For God is the creator of all things and who but He could define good and evil. Good is whatever is in accordance to the expressed will of God and evil is anything that opposes it. Because God says "You shall not steal" I know that stealing is evil and not stealing is good. Because God said "You shall not lie" or "Murder" I know that those things are evil and not doing them is Good. Because God said "You shall have no other God's before me" I know to worship Him alone is good, and to worship anything else is bad.
But what right does an atheist have to call something good or evil? By what standard do they call something good or evil? Is it there own subjective opinion? For if I proclaim to be an atheist, and I say "this thing that you have done is pure evil" you should rightly say "according to what, you?" Personal opinions are subjective. My opinion is not any more authoritative then the next person, so if an atheist calls something good or evil, who cares? It's just good or evil from their perspective, that doesn't make it actually so.
Indeed this worldview promotes what I would call evil based on God's truth and His revealed will in the Scripture. For if an atheist sees fit to murder, who is to tell him he is wrong if there is no objective truth about good and evil?
This is where some atheists will object and say, "Well, society defines good and evil. There has to be laws given by society or it would be utter chaos." Fine then. So why is murder wrong by this standard? Because the majority of our society deems it as wrong. But what if the majority shifted? What if all of a sudden 60% of people thought murder was a reasonable action if someone made you angry enough? By the standard given by atheism, this would then have to be called "good."
The problem is obvious. Aside from an omnipotent law-giving God, it's all subjective. An atheist has no right to call the incident at Virginia Tech or 9/11 or anything else for that matter "evil." Because to call something objectively, positively evil, one would have to go to an authoritative source outside of himself. Otherwise it is nothing more than an opinion.
Christians have a God of truth, He is the standard for good and evil. It is because of this that even if the whole world but me believes that murder, homosexuality, stealing, etc., are OK I can say "NO! It is evil because God said so!" I can call mass murder evil (including abortion). I can call stealing evil. Not because of my objective opinion, but because of the revealed will of the creator.
Can an atheist call something evil? Only if they realize it is either just their own opinion and holds no objective truth to it, or if they admit that there is a law-giving God and therefore lose their atheistic status.