That depends on the player I guess. If you be honest, every soldier or mercenary commits serial mass murder too.
I don't think very many soldiers are going to agree with that description.
So, if you want to address the topic of racism in a game without "dragging it down", the fundamental thing you have to do is have a clear, coherent, non-contradictory, and reasonable definition of what you mean by "racism". If you introduce a group with a belief system that is contradictory or non-functional and poorly realized for the game world, that's going to be at best comic relief. If you further insist that the belief system of this group is the morally right one via a DM blessing implicit or explicit, then yes that will drag the game down.
The only change that the philosophy I am thinking about would introduce is that belonging to a specific race would not count as a justification anymore for people following it.
So when this hypothetical cult of believes defines "a specific race" what do they mean by it? Do animals count? Animals in the D&D setting can talk ('speak with animals') and are presumably as sentient as the animals of myth and fantasy. Do plants count? Plants in the D&D setting can talk ('speak with plants') and are therefore sentient. If those races don't count under their racial definition, why are you allowed to murder plants and animals in order to consume them?
When they are talking about "a specific race", what counts - orcs, beholders, mind flayers, red dragons, vampires, or glabrezu? Are all of those treated equally? Are all presumed innocent until proven guilty? If not, then why not?
The belief system will need to seem plausible and believable. Either you will need to have a setting where everything is biologically and mythologically similar enough to humanity that it's plausible to suggest nothing is born evil and anything raised in a different culture could adopt the viewpoints of that culture - in which case you'll need to probably get rid of things like mind flayers, vampires, and demons. Or else, you'll need to have a setting where you have already established a completely different mythos behind pretty much everything. For example, you'll probably need to rid yourself of the various different types of dragons and of traditional D&D origin stories for dragons. And you'll probably need to come up with some sort of reason why dragons and other races war against each other other than that dragons are greedy, evil, engines of destruction that incarnate the violence of the natural world. For example, if you have dragons with Tolkien's Middle Earth back story, then it is literally nonsense to talk about dragons not being evil, or to act as if you are being moral by giving a worm the benefit of the doubt and a chance to redeem itself.
You can't pull that off in the middle of a game where you've already established certain truths.
Once you get to something like "orcs" though, if you haven't established a backstory for orcs, you can assert that orcs aren't inherently evil and that if you raised an orc with kindness and respect in a different culture that they could grow up with different values. I mean basically, Klingons are "orcs in space", and Star Trek pulled that switch without too much complaint.
If you are going to do that, try not to do it as a retcon (like Star Trek). One of the rules of a consistent campaign is that if it has a long history, nothing the players are likely to think of hasn't happened before. So work out what has happened in the past with orcs that were good, why it happens so rarely, or at least explain that it only seems rare to humans because mostly humans interact with the sort of orc that is a violent psychopathic bandit.
Either way, I used them just as an example for "sentient, non humanoid monster usually not attributed the same "human rights" than others".
Why in the heck would a living tower of flame have human rights? A fire elemental might have "fire elemental rights" accorded to it according to the dignity and nature of a fire elemental. One of those rights might be the right to life, which could only be forfeited in self-defense. But it's also not completely clear that a fire elemental is much more than the Plane of Elemental fire equivalent of a cow or a tiger, which is going to get you back to, do cows or tigers have rights, and if so what sort?
But it's not at all clear that a fire elemental has a right to liberty on the prime material plane. Maybe they have a right to live freely on the plane of fire, and I for one would consider it wrong to kill a fire elemental that was just minding its own business on the plane of fire, but if you acknowledge that a living tower of flame has a right to liberty and the pursuit of happiness on the prime material plane does that mean fire elementals have a right to burn? Societies that promote "equality" better think all these ramifications through if they don't want to seem silly.
That one can kill raiding gnome raiders just like you would orcs in that situation is pretty normal. What would change is that if orcs, or other non-humanoid monsters were living in a mountain not bothering anyone, even when they would show up on the evil radar, you would have to treat them the same way as you would treat reclusive gnomes.
At the heart of this, I'm not sure that this is a change at all. Like I can't recall a situation where the PC's were like, "Let's go somewhere where no one has been bothering anyone and kill what we find there even if they are peacable, just because they are orcs." That's not actually the situation that I find provokes the need for clarification. The actual situation tends to be the orc baby problem. If you find orc babies, what do you do with them. And that is the situation where a human society promoting the potential good in everything does actually show up and have a role, but it's also a situation where, if you suggest, "Yeah orcs are Neutral just like humans. They can be equally good or bad.", you might need to retcon your entire setting.
And finally, none of this has much to do with "racism". If you want to tackle racism in your game as a serious problem, you can do it much more honestly, more directly, and less problematically by having a "Quigley Down Under" type scenario where one ethnic group of humans (or dwarfs or whatever) hires the the party to kill the "bad" ethnic group of humans (or dwarfs or whatever). That way you aren't making unfortunate comparisons and you aren't being ridiculous by assuming that this thing that is so obviously inhuman still deserves human rights no matter what that thing is. Or at the very least, you won't get side tracked.
Not that that isn't fun. I've had a lot of great conversations and arguments about "werewolf rights" with my daughter. But "werewolf rights" doesn't really address racism, and it would be really unfortunately to make werewolves analogous to some real world group.
