What is the legal ruling on marrying a girl who has been engaged and had her marriage contract (aqd qiran) solemnized with another person, given that she wishes to annul this contract and marry me, and I am aware of the first suitor's bad character and the girl's desire not to complete the marriage with him?

You must repent immediately and sever your relationship with this girl. You have no way to her after her marriage, whether her husband is righteous or corrupt. Inciting a wife against her husband is a grave sin. The Prophet, peace and blessings be upon him, said: "He is not of us who incites a wife against her husband." Whoever does that has committed a great sin, and some scholars have gone so far as to invalidate the marriage of one who incites a woman against her husband. You must cut off your relationship with her out of fear of Allah's wrath and to preserve the honor of Muslims.