I saw this movie a few times and wonder why did Dmitri didn't love Katya? She was willing to marry him and willing to pay off his debt and attractive, why not marry her? She seems like she was desperate for his affection but he pushed her away and fell in love with Grusenka then Katya testify against Dmitri at the trial and became the scorned woman in the story. Also, I thought wasn't Grusenka was Dmitri father's fiancee before she met Dmitri?
---She was willing to marry him and willing to pay off his debt---
You're right, he could have married her to pay off his debts, or because she was pretty, and maybe even for the prestige of having a wealthy wife, but the whole point of the story is that for all of his money problems, at heart, Dmitri was an honest man.
Acting like he was in love with her and marrying Katya for superficial reasons, or to save his own a**, negates the kind of man he is. Same story with his father. His father loved Grusenka, but she only loved his money, and was in the habit of using men for her own needs. Dmitri ultimately was the man of strong character that she truly fell in love with.
Falling in love is a matter of the heart, and no-one chooses who they fall in love with. Getting married for the money is not a marriage, it's a business arrangement, and often ends in divorce.