Time doesn't allow for paradoxes. Have any of you heard about the multi-verse theory? That every time someone makes a choice, a new universe is made. So techniquelly, you may travel to the future, and kill your friend... And then catch up with yourself in the future, and watch as your friend died. Why you would do such a thing... I'll never understand... BUT if you traveled into the past, killed your friend, and then tried to travel back to your own time, you would STRICKLY SPEAKING IN THE MULTI-VERSE THEORY,You would not only be transversing time, but ALSO universes as well.