You know, talking about killing yourself in the past. (Got confused when Gerard started talking, so I'll start here)
Anyway, what I think is that if you kill yourself in the past, you de-materialise, nothing with you in it, you didn't exist, that would cause a downward slope in time to an alternative (lets say 2007). But if you are in the same place and time as yourself, that would cause a paradox, meaning you wouldn't be able to do anything and most likely again, de-materialise.
Going on about Time-Travel, their is no prove in the history books that it happened unless they disused themselves, if not, they'd most likely get burnt on a stake alive and people chanting, "WITCH, WITCH, WITCH!". But if it did happen, in the past there has been 'UFO' sightings, which could infact just be us from the future (I don't support this fact, just putting it out there).