At that point he's also arbitrarily murdering his allies and then trys to murder his only remaining family. He was already being possessed by duma when rinea gets to him. (
https://youtu.be/xCoquIV6vyk , he's already heard his voice before rinea gets there). Plus I have no idea how you can look at all of rinea and berkuts dialogue in echoes and not conclude they're deeply in love both ways. They couldn't have made it any more obvious, that's part of why it's so tragic. They even designed the map so that berkut and Rineas combined attack range draws a heart, as if they could be any more on the nose.
Plus he didn't deliberately burn his so for power, that was an unfortunate consequence of her following to try and help, but not being able to do anything to a duma possessed person, and duma taking whatever he wanted like the cruel asshole he was. Berkut had no intention of sacrificing Rinea, He then spends the rest of his time regretting having her die to get Dumas power, even in heroes.
