Silvia acted like the careless tunnel visioned meat head who didn't show him a shred of gratefulness despite him bidding for her, letting her join his party and not treating her as a slave.
Trauma and PTSD, it's not an excuse but instead taking the symptoms try to understand them.
It's like blaming a soldier that gets into panic mode whenever getting a stimulus.
And yes, slavery is bad m'kay, but just like you don't know Silvia's backstory, you don't know the slave trader's backstory either, but the direct parallel is obvious. It's their jobs to take care of their customers and they do so, they even handed over the elf slave to MC without taking money so they're holding up their contract at the very least and they can't save all the slaves because they're encumbered.
Slavers are slavers, they sell captured living sentient beings for money. They're people lost of compassion to anything living, they don't care if they make other a hell, they don't care if someone is tortured or raped, they want money.
/backstory
I don't think you're fine in head to stand for a slaver.
Compared to Silvia, who sacrificed all her allies lives, twice now, the first time she lost and became a slave herself (and any other elf who survived), giving them a horrible fate. The second time around they all for sure died and she didn't give a flying fuck about anyone, even willing to collapse everything and murder everyone, even any slaves that didn't get killed in the first volley of arrows.
The elves in this world are driven to extinction due do humans, they're hiding in forest and hunt as pray or captured for slavery.
They could either try rescuing anyone they can or be captured, die trying or die anyway but slowly as a slave.
If Silvia was labeled as a villain and we weren't dealing with slavery (m'kay), Silvia would be labeled as an out and out villain, if at the very least a terrible anti-hero due to her selfish pigheadedness and blind self righteousness (that doesn't even seem to change in the first run despite 10 years with the hero).
So yeah, the MC certainly isn't acting as a righteous hero in most senses (though come on with the goddess, two A's and a D and an F should've averages to a C, that's technically a pass so you should've just let him go home but just wiped his memory and shit) but I agree with this mentality of FUCK EVERYTHING, especially if it's going to be a never ending groundhog day bullshit with terrible people you're trying to be forced to side with or you die.
The stories of this caliber is either fetish or being able to see that doing something bad maybe isn't that right choice.