Experience is subjective. If you teleport to another location you're not the same person you were before you left, you're a similar set of molecules reconstructed in the same fashion. In all actuality, "you" died and another "you" took your place. But to you and everyone around you, it seems like nothing has changed, so you carry on with your life.

Likewise, if you travel back in time and change the future, you're not helping the people in the future you left, you're helping the people in the past avoid that future. But to you, it looks the same. It looks just like the past you're familiar with, and it develops into some semblance of the future you were expecting. So for all intents and purposes, from a subjective viewpoint you're avoiding the paradox and helping your ancestors. It doesn't matter if the people you left are still suffering, subjectively they don't exist anymore.