Why would the protagonist begin a monster be necessarily a bad thing? As far as I'm concerned, just turn off empathy and enjoy the ride.
But clearly that's not the common reaction, as villain protagonists, truly villainous ones, are incredibly rare, so I guess most people can't do that in the same way.
Mainly because Villains aren't all that easy to make engaging as protagonists. We get to see them
too much.
They usually run into 4 problems
1. Humans tend to usually become attached for the smallest reason (i.e see Aura from Frieren becoming a top 10 favorite character after appearing for like 1 chapter/1 episode)
People don't like reading a story with characters that are gonna unceremoniously die in like 10 chapters because the protagonist kills/enslaves/whatever evil deeds everyone especially if those characters are designed to be liked.
2. People don't really want to spend their time with something that constantly loses engagement.
For point 2, it's because of how the Villain protagonist usually operates. I mean come on, let's not kid ourselves here, a truly villainous protagonist will require the author to haul ass and lock in when writing the other characters in that world. Usually, they also need to make the actions of the villain have consequences.
Otherwise, it becomes
boring, there's fun in watching videos of pro RPG players, but they aren't really
mainstream.
That has a problem of running into point 1. And even if the author makes it so that the villain protagonist gets their comeuppance-
3. The people who
like a villain protagonist that way are usually in it for the power fantasy and
hate seeing them lose. Especially if it's stupid.
This one is particularly bad. Mainly because most people who write villainous protagonists usually write them
too strong and get stuck in a corner of needing a literal Mary Sue to beat them sometimes.
And yeah that comes to the final point.
4. We see assholes in real life manipulate the system to shit on people too often. No one is gonna be able to just
turn off their empathy when they already are getting screwed over or are in a similar position.
And if they can, you run into problem 2.
These reasons are why you don't usually see a villainous protagonist become mainstream. And also the reason why it's
way easier to write a fanfic with a villain protagonist or a story that utilizes already existing characters.
And I'm assuming we are talking about the VILLAIN protagonist here since you mentioned a truly villainous protagonist. Someone like Loli Hitler from Tanya the Evil or Adolf Ooal Gown from Overlord.
And even then those two series have their problems too.
Not saying it's not possible, but just saying that it's not gonna get that big outside of the power-fantasy crowd. Who will literally gobble up even the dumpster fire series so the effort isn't as needed.