Who killed Silas?

When I first heard that Michael Easton was leaving the show, I figured Silas was going to prison. I thought Silas would confess to taking the baby to save Ava. I thought they would haul the good doctor off in handcuffs.

Much to my disappointment, Silas was murdered. The question is – who did it? I don't believe it was Nina. Even though she was hopping mad, I think Silas was