Maybe it was a success. Maybe we didn't send a person back, we just created an explosion in the past for the purpose of verifying that it altered history. Success! Now we just have to find Nelson Mandela and the creator of the Berenstein Bears...
(Okay, I'm done being a crackpot now, it was fun though)
How many timelines are there? Or is there just a single timeline and the act of sending this event back in time has always existed, the timelines conjoined at some quantum level.
It feels paradoxical either way to me, but maybe the multiverse is cool with there being universes that are paradoxical.
Perhaps there's a dimension beyond time, and just as information changes in space across time, the entirety of our timeline changes within this upper dimension. The timeline that we once thought was straight and obvious turns out to be wiggling around and looping through itself in this "time of times".
Wouldnt be easier and safer for timespace to just drop an artifact in remote area and then try to retrieve it though? Killing people as an timespace event could cause changes in to the world.