slavery can be an act of mercy.
Not really, because there are many other things that that person could do that would be Merciful. Rather than Claiming another human being as their property, they could adopt them, or take them as a ward, or look out for them, or any number of things that would be Infinitely better then Slavery.
murder can simply be a day at work, or even a commendable thing.
In this I believe you are talking about Soldiers.
Good Soldiers Kill, Bad Soldiers Murder. A Soldier kills because he or she is forced to, in the name of Self Defense or Defense of the people who cannot defend themselves against their aggressor. If you must Kill in order to stop others from Killing then it's not Evil. However in the chaos of war, sometimes this gets grey, accidents happen, blood gets a little too hot. What turns killing into murder is taking away that need to kill.
even *insensitivity* has in some societies been a fully acceptable part of life. i'm fairly sure that the vikings didn't consider themselves evil when they attacked, killed and/or *insensitivity'd* the citizens of various villages and cities. the english noblemen likely didn't consider themselves evil, when they *insensitivity'd* newlywed women in taking their 'Right of first night'.
Vikings didn't consider the people they *insensitivity'd* to be human... it was more like bestiality to them, but not that bad because at least they Kinda looked like them. Even that's an Evil POV, as they were Humans, forcing themselves on other Humans, regardless of how they tried to justify it.
What the English Nobles did however Was Genuinely Evil. They attempted to literally Breed Out the Scots with that horrible law. That was their goal, so they made *insensitivity* Legal. There wasn't anything high and good about what they did. And I bet a fair amount of people thought the same.
torture is another thing that is considered evil, but it has, and very well might still be, an acceptable way of extracting information from prisoners.
Torture Never Works
You can do anything to them, and they will always say what you want them to say, regardless. If they aren't saying what you want them to say then you've not inflicted enough torture.
Saying that you need to extract vital information from a prisoner through torture is a Weak justification for it, as the validity of the facts is highly in question. Not only that but look at it's history, to see how it's abused. All the way back in the Spanish Inquisition, through even to Nowadays at Guantanamo. You know how may Innocent people they tortured?