Oh, that's actually a pretty interesting question! Here's the thing: your players probably already think killing people is bad! The thing is, the word "people" is carrying a lot of weight there. For example, in shmup videogames (where the point is the excessive violence) usually the target of your killing spree is some caricature of a group the devs don't count as people, such as the KKK or WW2-era Nazis. This is also why modern shmups take place in the middle east and have you gunning down Al-Qaeda or ISIS or some other dehumanized boogeyman. It's not generally a distinction made consciously, and everyone sets the line differently. Lots of people I've met on Hexbear or Lemmygrad don't think cops and alt-right nazis are people, which I disagree with. And most people I know IRL definitely believe their pets count as people, which I understand is controversial.
My point here is to point out that we all at some point decided what counts as a person, and it's a touchy opinion that rarely gets examined. A less touchy equivalent is how many people have very different opinions on what counts as cheating, but are convinced that their opinion is objectively and inarguably correct. I think it's worth examining what is a person to you and why you decided that.
However, as a gamemaster you have to allow your players to make two choices:
- Are the monsters we are fighting people or not?
- Does my character agree with me?
Taking these choices away from them is not fun. However, if you want to encourage a particular outcome, you can put a finger on the scales through game design.
Old-school dungeon crawls dealt with this by making combat Not The Thing We Are Here For, since the players are playing as professional graverobbers who are here to hoover up anything that isn't nailed down and get the hell out, since you only gain xp for the gp value of the treasure you loot. If you have limited time and resources, Combat is a needless risk to be avoided. This usually results in players actively trying to negotiate with the sentient denizens of the dungeon for mutual profit.
A more narrative approach is to have the players be a part of a society that has opinions of orcs and goblins that mirror colonial-era America. If the players notice that their fellow citizens talk about dealing with goblins in terms of extermination and population control, they're probably going to have a "Are we the baddies?" moment unless they are either very dense or racist (and having played at a table with both, the difference between stupidity and racism is very obvious). It's also pretty cool to play as a freedom fighter, and a lot of groups will gravitate towards fighting against colonial oppression—but only if they feel if that choice is non-obvious and therefore they made the choice themselves; if fighting the power means opposing YOU, then thats what they will do, and we dont want an adversarial table, right?
Nah I'm in my 20s, that's just what happens when you are super horny, aren't ashamed of it, but also don't know what sex is.