@Madcat6204
"mens rea" means "guilty mind" in Latin. In law, it is the mental element of a person's intention to commit a crime - i.e. the knowledge that your actions (or lack thereof) will cause a crime to be committed. It is a necessary element of many crimes - including murder.
So when the judge said, it is "Hard to ascertain whether there was mens rea", what he is saying is that the MC did not intend to kill her fiancee - i.e. it was an accident. And if it was an accident she did not have the necessary "mens rea" for the crime and therefore cannot be found guilty.
Hopefully that answers your question