1. morally wrong or bad; immoral; wicked: evil deeds; an evil life.
Unfortunately, the preceding definitions don't make it entirely evident what "evil deeds" might be. Of course, when the issue of "morality" comes up, this is virtually guaranteed to create controversy, since there are conflicting opinions regarding the morality of various acts. For example, it is generally accepted that killing is immoral - unless you're killing the "enemy" for your country, or the heretics, infidels, and apostates identified by your religious denomination. Thus, it seems that even the believers in moral absolutes find it possible to violate the dictates of their nation and/or religion.
Whenever the subject of "evil" comes up, the name of Adolf Hitler eventually intrudes into the conversation. Perhaps Ioseb Besarionis dze Jughashvili ("Stalin") might also surface in the discussion eventually. Most everyone would agree that these men were evil incarnate - together they combined to cause many tens of millions of deaths, with Stalin accounting for more than Hitler, actually. I could go on naming people (mostly men) whom most would agree were evil: Charlie Manson, Jim Jones, Kim Il-Sung, Pol Pot, and so on. Of course, such folks also had their proponents at the time. At the time the deeds were being done, these "evil" people were perceived (at least at first) by their proponents as doing good things. (I'm reminded of the saying "Extremism in defense of Liberty is no vice!") Such proponents might say their leaders were just a bit too carried away with enthusiasm for their programs, but they weren't evil programs! Evil deeds are often justified as being necessary, despite their cruelty, in order that good will come of it! Historically, this is the argument of all evil people.
Many evil deeds are done by people who believe themselves to be good, because they feel they're in the right and their actions are justified by the good ends they serve. It's often said by evil people that the end justifies the means, after all. Such people don't imagine themselves to be evil, but their deeds proclaim it! Evil people almost never see themselves as doing evil. Rather, they believe themselves to be a force for good, willing to do the unthinkable to reach some goal that makes their evil deeds worthwhile.
Thus, most evil deeds are committed by people who have some cause that renders justifiable any action, no matter how evil. Murder, racism, slavery, torture, rape - all are done by ostensibly good people who have rationalized their evil actions. When the acts become horrific enough, we decide that such people are criminal sociopaths - criminally insane. Unless, of course, they're our national and/or religious leaders; in which case, the adherents line up to bathe themselves in the agony being inflicted on others, to show their unity behind the cause that justifies the deeds. Or they stand by silently, saying nothing, perhaps for fear of becoming victims themselves or perhaps because they can't decide what to do. Massively evil deeds require the active or passive cooperation of many, many followers, as well as the evil leaders. An evil leader can carry out his evil acts only with the explicit or implicit cooperation of his nation (or sect). At the Nuremburg war crimes trials, we repudiated the notion that "I was just following orders!" is a valid excuse for perpetrating evil deeds ... or did we?
So long as people choose to let demagogues and fanatics make their decisions for them, do their thinking for them, and determine their agenda, such people are liable to commit evil deeds, thereby becoming evil without even thinking about it. It seems in today's world, we have vast numbers of demagogues and fanatics on all sides. In a war between opposing fanatics, moderates will be forced to choose: with us ... or against us. What choices will you make?