Practically every point made here is something I can agree with, which just makes it even harder to pick only one of these acts as the worst. Now, I've had no personal or second or even third hand experience of any of these things, for which I count myself very lucky. (which is probably a sad state of affairs in and of itself) So, all of the following is nothing more than conjecture and opinion, obviously. And there are a huge number of variables that have to be taken into account when trying to decide which of these things is the utter worst, when we are all aware that any of them isn't awesome, at all...
In the end, however, I went with slavery. Because it can readily include both rape and murder without repercussions, which is a pretty big deal as far as laws go, but also because it goes above both by effectively ending the slave's life while still leaving them alive. And even if they are liberated and do win free and recover, they still have to live with the knowledge that their life was taken from them, that someone had all that power over them and that they allowed it by being unable to stop it, and others who could, didn't. If they win free through the abolishment of slavery, I don't know, maybe it would be easier, but if they win free and slavery is still a thing, then they'd also have to be afraid of it happening again.
I still have a little bit of trouble with this argument just because I know that a lot of rape outside of slavery can go unpunished and that people being afraid to speak up about it means that having laws against it or not doesn't change the stigma. And the victim would likely be afraid of it happening again and would take a great deal of time and effort to recover, and the recovery process might include feeling capable of living their life the way they want to again. It could go so far as to be internalized as a I am less than they are feeling, which is certainly not pleasant especially after having been traumatized.
And murder just straight up ends everything. But as that's the case, I find it more bothersome by way of the life that's been stolen, as well as the lives it affects beyond the person who died.
Beyond the effects these acts each have on an individual level, however, they each have some affect on those around the individuals in question.
Slavery still wins on this point, because on one hand it's probably subjugating a huge number of people (otherwise it wouldn't really be beneficial for the economy or whatever grand scheme is being hatched, or just effort wise, one slave does not a great empire make, and if you've got one, you're likely to have more eventually) and treating them as lesser beings, and on the other hand, it is teaching all those who aren't slaves to see themselves as the superiors, and to look down on any one who isn't part of the 'us' category. Which can be a pretty damaging point of view by itself.
Rape has a slightly smaller range. But as can be seen from a lot of reaction to sexual assault trials, how authority deals with rape can have a huge impact on how safe people feel beneath that authority, and how significant their voice is or isn't. And how freely someone might take advantage of a lack of action on that authority's part.
Murder can have political ramifications. Close family and friends will be hit hard. Martyrs might be made. It's hard to say just how many people would be affected by any one murder, from whole countries/religions/ethnicities to the one person who knew that guy and the people who had to clean up the body.
So, yeah. I definitely think that it's not easy to say any one of these acts is worse than the other in every single situation, but I do have to say that slavery strikes me as more likely to have a detrimental effect on an individual and broad scale, more often than the other two.
