Motive: Frank doesn't necessarily need a motive. He's proven he'll do whatever he's told. If someone told him to dispose of Rebecca, there's a chance he'd do it.
Evidence: Considering Frank is the one who kills Lila, it's easy to see how he might murder Rebecca as well. Plus, we're not sure if Annalise knows Frank is Lila's murderer. This explains why he'd want to cover up his involvement in Rebecca's murder too.