My question is for someone I know well. She is guilty about something that happened to her. She used to live with her family in her cousin's house. There her male cousin used to treat her extra nice and thus she thought he was being nice to her as she was young and very very naive and innocent. She was unaware that he was trying to seduce her as she was very innocent. Time passed and he got very far with her without her realizing it and she just used to like him a lot and once he asked her to promise that she would marry him and she did agree without knowing at the time she was young and naive and didn't know much about Deen. When she grew up and Allah guided her she realized that he was a bad Muslim and made her think he was being good to her whereby he was only using her. Then she stayed away from him and repented to Allah for all the sins committed with him. Then they moved away from that house and she became a very good Muslimah, now who doesn't talk to men and stuff. However that same cousin of proposed to marry her and told her family they were in love and wanted to get married. But the girl who changed and realized what a bad Muslim and sinner he was refused and declined his proposal because she wanted to marry someone who had good character. She had changed but then he reminded her of the promise which she made when she was young and naive and didn't know the Deen at all. Is she sinning by breaking that promise and refusing to marry him because she feels guilty that she had made a promise and didn't marry him? Sorry for the long question I really hope you can answer it.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

If the first person who proposed to you is of ill-nature and commits sins then you should not accept his proposal even if you have promised him previously. Fulfilling a promise that leads to the violations of Shariah is not compulsory.
Therefore, you should marry the person who is pious, and has pleasant character.
Allah knows best.