Sure, but what if that doesn't happen? One of the issues with attempted foreshadowing is that you, as the DM, have no idea whether something will actually happen in the future. Time travel is just one more avenue by which someone might learn the outcome of events which have not happened yet.So if/when that happens....the Mercenary could kill the Fighter. So each of them is the other’s killer. It’s the kind of whacky concept that can only come up when time travel is involved.
I guess you'll have to wait and see, but there are many more ways for this to go wrong than for it to go right.