If Oswald didn't shoot Kennedy someone else will have.
Your edit provides the context for this very bizarre statement.
They're employing the future perfect tense into the passive voice and doing it to convey a sense of possible different outcomes.
That's a mouthful. So, instead let me illustrate it for you:
Assassination plotter (AP) A says: What do you think happened? Do you think Kennedy is dead?
AP B says: We hired a bunch of shooters for this job. If Oswald didn't shoot Kennedy someone else will have.
So, we know that there was a planned assassination attempt. We don't know for sure that it went off correctly. But, we do know that the assassins had a backup plan.
If (the condition arises where) Oswald didn't shoot Kennedy (by the time of the planned assassination -- indicated by the past tense didn't shoot) someone else will have (done it instead of Oswald indicated by the future perfect tense will have).
This type of difficulty parsing the statement is why many style manuals warn against using the passive voice.
You could make this whole thing clearer by saying:
If Oswald did not shoot Kennedy, one of the other guys did.