Spoilers below please don’t read unless you’re willing.
I really doubt it’s Jack since it seems he doesn’t really know the mansion well since he needs to “check out the place” and that would be a weird thing to say in my opinion if he knew where the Library is as well as opening the second vault to kill Oswin. I believe he’s just a random mercenary sent by Orlando’s father and considering the second voice on the radio he’s not the only one. Also Dave is the one marked for death according to Benson, probably due to what his father did which is supported by what Thanatos said. I think it’s more to do with Dave and David. Whether Memphis has a hand in it besides assassination and robbery is unknown to me at the moment but that’s what I got.