In Richard The Third, Why do you think Anne finally gave in to Richard when he asks her to marry him? After all the bad things she said about him?
Although Anne despises Richard, she gives in and marries him because she needs to be aligned with a powerful man. She accepts him, she says because she is weak and succumbs to his charm, but she knows that he is a murderer.
In this period in history in England, murder, mayhem and plots were common. Anne probably thinks she will be safer if she marries Richard because her position is uncertain after her husband is killed.
Also, it is logical in royal circles to remarry, quickly, especially to an heir to the throne, like marrying your brother-in-law to continue the succession.