Would the woman who asks be sinful for leaving a man who claimed he would resort to God with a complaint, after she had returned to him fearing God's punishment for the harm she had caused him, then separated from him due to her inability to love him or accept him as a husband, especially after he made statements about his intention to commit indecency before marriage?

You have done well by repenting from lying and from communicating with the young man, and you must be careful not to revert to such behavior. You are correct in leaving him and cutting off the relationship, and there is no sin upon you for that. It is not permissible for a Muslim woman to be in a romantic relationship with a non-mahram man. A Muslim woman should seek a righteous husband who possesses good character and religious commitment, because the Prophet, peace and blessings be upon him, said: "If there comes to you one whose character and religious commitment you approve of, then marry him. If you do not do so, there will be tribulation on earth and widespread corruption." Al-Hasan al-Basri advised marrying a girl to a man who fears Allah, because "if he loves her, he will honor her, and if he dislikes her, he will not wrong her."