Mutants.
No one in our reality could travel back in time. Until someone did. They created robots that could identify a genetic line that might eventually produce mutant offspring. In a time when mass extermination was rampant throughout the continents of both Europe and Asia, when millions were slaughtered and buried in mass graves, how difficult would it be to hide one more act of genocide against a class of persons that did not yet exist? It would only take one person to reshape the world. Our blood would move the wheels of history.
His assassination served as the tipping point. But when we saved him, it wasn't enough. It only strengthened his resolve to use the protocol against us.