Who told the story of Tessa pushing SBK downstairs anyway? Has to be Maddie, right? If it was Maddie, it could've easily been Maddie killing SBK and then pinning the blame on Tessa. If she's strong enough to strangle Morales, she's strong enough to push SBK downstairs.
If we want to go even more Machiavellian: SBK is a hitman hired by Maddie for some reason, something happens that causes the connection between Maddie and SBK to potentially leak (maybe SBK is about to leak the connection himself, either carelessly or on purpose), so Maddie "hires" SBK to come in and off a family member (Mitchell?), with the intention of betraying SBK and killing him in the dark, thinks she succeeded, pins the blame on Tessa, manipulates Mitchell into killing Morales, and Garrett into burying the body. Totally far fetched, but at least it would make SBK more than just some random dude that finally shows up in episode 11 and is immediately offed by a sickly little girl.
I think the consensus now is that the accomplice is Sophie and/or Tom? I am on Team Sophie, but Tom is certainly the next most likely possibility for me.