Unfortunately the term slut was how r*pe victims were referred to in this historical context :( so I don’t think the ml used it in that way. Rather stating that in the past life the mc would have been considered a “slut” by the village even though it wasn’t the mc’s fault. Which is probably why the mc struggled with lust in his present life because r*pe can often manifest into unfulfilled sexual desires/ emptiness.
I can see your point that mc would have been called a "slut" back in the day for being fucked by multiple men without taking into account if he ever consented.
And that ml lived a very isolated life so he just used the term he had picked up back then.
But it isn't like the word "slut" was a neutral word in the past and then the meaning changed over time and it became offensive. (Though it might be different for the original Korean word that was used, I can't say anything about that.)
Back in the day it was absolutely meant to be offensive because it was describing a person whose behaviour was offensive.
Someone who had sex with multiple men was considered the lowest of the low and even unclean in many places.
What did change over time is our understanding of consent (and gender norms and oppression through conditional "protection" based on purity)
If the word "slut" describes a person who has a lot of sex, then it can't be used to describe someone who has been raped, because they never consented to having sex in the first place.
The same way you would not use the word philanthropist to describe someone who was robbed at gun point.
That means that no matter which way you look at it, in English (again, I can't say anything about the original Korean) ml used a derogatory word to describe the mc, who he claims to love.
And that is something that doesn't sit well with me (even though I can forgive the non consensual somnophilia, because that's different

Because wtf do you mean telling him that he was a slut in his past life while knowing that he was imprisoned, gangraped for months and then murdered by his rapists???
Is this asshole serious?