You have to think like a Soap villain. Killing Jenna would get her out of the way, but also risk turning her into a "martyr." She becomes a precious memory from Bobby's past, that might make him feel wistful over what could have been, possibly regretful that he didn't fight harder for her when he had the chance. Framing Jenna for murder, on the other hand, not only gets her out of the way but also tarnishes her memory. Now instead of "the one who got away" she's "that psycho ex I'm glad I didn't stay with." So, in that context, it was a rather smart plan.