Time travel and consistency are like oil and water: you can try to mix them all the way you want but at the end, they always end up separating themselves. This is true for science-fiction as well as for real physics and you can't do nothing with that. If you think that you had achieved a consistent view of time travel in a science-fiction serie, then it's because you have made a serious error somewhere but you haven't noticed it yet.
For example, if we talk about the millions of Vulcains killed by Nero, we are in an alternate timelime; therefore, these people are not the original Vulcains but copies who have been made in the alternate timelime in order to be killed but the originals remain there; untouched and unaware of anything particular.
It's like to want to kill you but instead of killing you, I make a copy of yourself - in a way totally unknown to you - and I kill this copy. You, in the meantime, you remain 100% unaware and unaffected of absolutely nothing. I could do this 1000s of times, you would remain exactly the same, both physically and mentally. The real clown here would be me, losing my time and my energy with copies of yourself that even you remain totally unaware of.
Furthermore, if travelling in the past create a new timelime, travelling into the future create a new one, too. Therefore, once you start time travelling, you cannot no longer coming back to your starting point, travelling instead to a new timeline everytime that you try; each of these timelines filled up with copies of all the people existing in the previous timeline. Who would care about the destiny of anyone in any of these timeline in this situation?
If time travel can really exist then destiny doesn't exist anymore.