I knew pretty quickly that I wouldn't say murder is the worst evil, just because there are so many gray areas to it. Like, I think we can all generally agree that murder is wrong, but then the question becomes, what counts as murder? Is the death penalty murder? What about abortion? Self-defense? Killing an enemy soldier during a war? Obviously it would be easy to imagine an example of murder that's pretty far removed from any gray area, but, it feels weird to say "killing another person is wrong and therefore evil in any circumstance" when there are circumstances in which it stops being so cut-and-dry and instead becomes a debatable subject. So, I can't see it as being the "greatest evil".
I had a harder time deciding between rape and slavery, but eventually what made me choose slavery was this: while rape can be an incredibly traumatic experience that can really fuck up your life, at least you have your own life to return to. You might feel broken, you might feel like you can never be your own person again -- but the truth is, you do still have your own life, and you have support you can turn to, and it's possible to recover. I'm not saying it's easy, and I'm certainly not trying to trivialize how difficult it can be for many rape victims to recover. But my point is, it's still possible for you to recover, to be your own person, and to go back to living a fulfilling life. If you're a slave, however? You don't have your own life. You are literally the property of another person -- and your entire existence hinges around what your owner(s) want to do with you. This not only means that you might have to endure some of the shittiest treatment imaginable, depending on how cruel your owners are -- but, even if you're relatively well-treated, the point remains that you simply cannot go out and live your own life. Your life is merely an object that someone else owns, and that remains true for your entire life. Rape is an awful experience, but, hopefully, you won't have to suffer through it again -- a slave, meanwhile, doesn't have the same hope for "recovery", doesn't have the same light at the end of the tunnel. A rape victim is still a person, who has an identity outside of being a rape victim, and who can go on to accomplish other things -- but a slave is just a slave. They are, in the eyes of a society that allows slavery, not even considered to be "a person". They are an object -- an animal, at best. A rape victim may feel like they've lost all their dignity, they may feel like their rapist didn't even treat them as a human being -- but at least they are still given the dignity of being considered a human being. A slave simply doesn't have that. And it isn't just because of some traumatic experience that one awful person inflicted upon them, either -- it is a fact of their existence, one that they may have even been born into. And, in order for slavery to even exist in a society, that society must have a decent number of people who approve of slavery, or else the practice wouldn't be able to exist -- which means that society in general does not give slaves the dignity of being considered people. It isn't just one rapist taking away their dignity as a human -- it's their entire world, and all their peers, refusing them the dignity of being called a human, for life. And that, I'd say, makes slavery more evil than rape.