Only one claim we've discusses so far has deserved an ad hominem from Elliot Sober, the author of my philosophy text book: that evil doesn't exist. Sober discounts those who make the claim as largely the most privileged, who have never had the opportunity to see evil. He tells readers inclined to the view to think about how they would feel if they had maybe actually experienced hardship. Sadly, that's two fallacies right there: even hypocrites can have accurate reasoning, and emotional responses make very poor reasoning. They are called emotions, not rationals, because they are irrational.
My teacher was equally dismissive of the claim. He responded that evil appears to exist. I'll grant that. But miracles and religious visions make God appear to exist too.
The problem of disproving evil is to do so without marginalizing the experiences of those who believe they have experienced it. Obviously, tragedies take place on personal, community, and global levels.
Philosophy loses out from fear of speaking though. Even poorly reasoned arguments need to be voiced for analyzing and refuting. While marginalizing opinions and experiences should never be philosophy's goal, it will result from conclusions. Someone's experiences will always contradict that conclusion, especially in a subject like philosophy of religion. Concluding God doesn't exist negates the experiences of the religious, concluding evil doesn't exist negates the experiences of victims of tragedies, and so forth. Many of the largest questions, and most pervasive and powerful potential answers are in philosophy of religion.
I still remain unconvinced that evil can be proven to exist. I see a linguistic game here. The evil to actually negate God, as from the Argument from Evil, would need to be extreme. I think we call the worst of wrongs evil, without regard for the actual line between wrong and evil. Punching a friend may be wrong, but few would say evil. So, I remain uncomfortable with the claim that evil obviously exists because we see extreme wrongs. Compared to the power of creation, to fully oppose that, evil would be enormous. With God, the scale can easily become incomprehensible. Real evil too could be incomprehensible, and that little bit of doubt, to me, means that any argument which relies on the existence of evil is weak.
For all that, I am apprehensive. What I have seen so far has been an almost uniform assessment that evil must exist, if only to avoid trivializing tragedies. There's such a limited path when discussing God, because so many easy conclusions lead to an inability to discuss further. So, I'm writing here, where I figure at least most people who see this will either ignore or actually look at the full argument rather than the single conclusion that the existence of evil indeterminable.
"Evil" can really mean anything from extreme wrong to innate desire to oppose "good", its colloquial use is very free-flowing, so there is a lot to be confused about by it linguistically (or else I'm just easily confused). Supposedly its original meaning was closer to what we'd associate with "cruel" or "harmful," which is nicely enough more concrete. To me, though, even if you're using it in one of the less dramatic contemporary senses to mean something extraordinarily immoral, in order for the meaning to have much reach I would still need to be convinced of the existence of a more or less objective moral system, which is a whole discussion in and of itself. Maybe if it were taken to mean something like "innate desire to cause pain or behave cruelly" I could see it having traction. I don't think it's usually used that way, though.
I mainly avoid it in speaking and thinking because I find that it has the net result of dehumanizing very human flaws and misdeeds. I also think that that's part of the concept's appeal when you get right down to it. Using it often allows us to put distance between our idea of ourself and the nasty things a meaningful class we belong to (person) is capable of. The word and its various definitions/uses are more a vehicle for that distancing than any one fixed thing most of the time, from what I see, so unless distancing is part of the goal of the discussion I think its usefulness is usually limited, provable in some way or not. In a way I think the idea of it tends to trivialize tragedies more, by implicitly trivializing our relationship(s) to them and their frequent causes.