Karen curse cannot be broken from true love but from getting pregnant and then the curse passed in to the child thats how her mom cursed was broken and passed into karenÂ
The duran killed her Because he wanted her to live forever. He viewed her as his g**. The way to break the curse is to have a daughter to pass that curse down to so he would want to kill her before she could have any child. He also gave her the coin so he can tell she kept regressing because the number keeps increasing. While he may not felt or experience the regression, he knew from the hints he planned beforehand and what the mother told him. Honestly he wasn't "bad" and that's why even though he was the one who killed her, he wasn't the villain and the villain of the story ended up being the prince. FL forgave him at the end and he managed to find a cure for her, but it took a long time for him to realize what he did was wrong. He didn't know her regressions broke her and thought it was a great power.