Perhaps Marissa new she was the twin of Babe and she is the one that meant to kill her but accidentally killed Dixie?

I completely agree with this. Ever since it was hinted that someone intending on killing Babe was NOT Cambus, I thought it was Marissa. I'm now thinking this was a way to write her out if she didn't work on the show. I think it would be interesting if she also intended on killing Adam because he was aware of her existence, but kept it a secret; and killed Stuart by mistake. Now she is trying to stay close to the case in order to avoid being found out. Just speculation at this point, but I hope they have a big plan in store for these events!!!