@DrPie He killed her “family”... you remember, the ones who kept her locked up and abused because she was an illegitimate child born from a commoner. She feared for her life when the Prince invaded, but it isn’t like those people actually treated her like family.
Also, the Prince explicitly went there to find her and make sure she made it out safely (though she still doesn’t know that), because he had heard of her reputation of helping the common people. He wanted to ensure she didn’t die, and hoped to marry her to bring stability to the kingdom.
There’s still a lot we don’t know about the Prince’s background and why he attacked the people he attacked, but it’s obvious that he’s not as bloodthirsty as his reputation suggests.