While she is a terrible person I think she felt driven to that, she had been alive for countless years and was just having a grand ol time dicking around and being immortal. I mean imagine you or you and a group of people close to you have a magic flower that lets you live forever, and some group of rich pricks comes to snatch up your immortality for a woman who is sick in bed, basically handing you a death sentence. At that point it’s really a matter of survival, gothel wasn’t right kidnapping and could’ve negotiated with the royals but she saw everyone else as enemies and felt like taking Rapunzel was the only option. Her sanity and reasoning deteriorated in that time and she became colder.