Fort mode also clearly allows for really despicable acts as well, just on a larger and less precise scale. I.e the Mermaid Genocide incident (though to my knowledge the community didn't have THAT much of a problem with it - I don't fully know since I still wasn't around on the forum when it happened) or that one guy who played his fort like an aztec temple.
I think when you get in-character with the "dorf mindset" you do sorta tend to get carried away with doing things that don't really matter for anything besides sating your own curiosity (i.e finding out if its even possible, or if it is then finding out the most stylish and/or inefficient way to do it, or just finding out if the cost is worth it).
Technically, it wasn't a genocide, it was ranching. It was an elaborate way to replicate exactly what people do with, say, pigs, but with nominal sentients. Also, it wasn't actually done, merely suggested, before Toady changed mermaid values to make them less attractive, only later on with sea serpents, did Sphalerite actually go through with some of the ideas. (But people were more focused on the engineering, then, rather than the fridge horror.)
Beyond that, people do this stuff because it is a game, and they are simply inflicting damage numbers on "e"s. It's fascinating for being able to "break the rules", exactly the same as people trying to find the "You weren't supposed to see this, you know" in Grand Theft Auto 3.
It's up to everyone's own personal level of empathy with the game to determine whether they feel they've actually transgressed against someone, or simply advanced a game objective. I'd point to both
DEFCON, in particular as a game where inflicting unimaginable casualties is both a game goal and also kept so utterly remote from the player that millions of deaths are literally treated as scores popping up on a game-like screen, versus, say, Heavy Rain's visceral amputation scene where the game is clearly designed to make us feel empathy with the character in particular. (And that's leaving out any FPS game that just revels in blood and gore...) The "closeness" of the player to the suffering in the game directly impacts how a player is prompted to feel about those events, and in DF, violence is described to us in strictly clinical terms after the fact. (The axedwarf bashes the goblin swordsman in the second knuckle of the fifth finger, left hand with the *pine wood shield*, bruising the bone.)
Lastly, I'd say partially some of the things you mod in also say a bit about your mindset and/or interests, not just in term of what the creature actually does but what it is appearance-wise.
Myself, I have to admit that most of the non-playable creatures I modded in are giant murderfucker beasts - even the only benign animal I made can still kill you indirectly or atleast make you go crazy, should you eat it's meat or drink its blood. Obviously playable races don't count since they have both normal, benign people and also violent bandits - some have more of the latter than others.
Sounds like a standard "challenge" player to me - you see DF as a game of challenges, and when the game is "too easy to be fun", you add in bigger monsters to make combat "more satisfying", because,
combat is where you get your satisfaction.
Meanwhile, I'm procrastinating on modding in various syndromes to give effects from various herbal teas, including the likes of Meadowsweet, AKA. Meadsweet, the naturally-occuring weed that is the source of the chemical compound for aspirin, as well as an ancient sweetener, and am giving it painkiller functions. I'm mostly doing it because, WHEE! research and learning are fun! Now, let's put these things I've researched into the game! Like I said before, I don't think of challenge or combat as an important part of the game for me, but at the same time, lots of people clearly do.
So, from my interactions thus far. It tends to be science oriented individuals who are not into socializing, partying nightlife, Instagram/twitter, sports (larping, E-sports, and chess not included). They usually like things like reddit, computer science, physics, and sometimes cats (I think cats are ok).
Which is the stereotypical way of describing pretty much anyone into gaming, isn't it?
Surprise! People who play heavily-involved single-player games, which requires technical, logic-oriented thinking and focusing upon a single project without constant socialization attracts people who do not require constant socialization and prefer logic-oriented thinking. That only puts DF players in the same box as every strategy game ever made.