But we killed him in LOZ when he had the Triforce of Power. I know that the game is old but, it still doesn't change the fact that we actually killed the bearer of the Triforce of Power.
Well you can look at it simply, or in detail. If you take it simply, those older games didn't have a very developed storyline.
Looking at things in detail, LoZ is not the only game that Ganon or Ganondorf has been killed while bearing the ToP. The two weapons that have shown to be able to defeat and kill Ganon/dorf in any Zelda title has been the Master Sword and the Silver Arrows. In LoZ, the Silver Arrows were used to destroy him, even with his possession of the ToP, and in ALttP they were used again to destroy him, even though he did not bear the ToP, he had the entire Triforce at his disposal apparently in the back room of the Pyramid.
Then we have the newer game that is Twilight Princess. Once again, Ganondorf had possession of the ToP, and by the looks of things, died. My guess as to how this happend was the prolonged impalation of the Master Sword. Link in OoT merely stabs Ganon for a brief moment, and he is able to survive with the ToP intact. However in TP, Link stabs Ganondorf straight through the guts, leaving the sword there to apparently fester up all of Ganondorf's evil powers, as well as pulling the ToP from him (as it is shown to loose its glow on his hand).
My point being, the ToP keeps Ganondorf alive as long as neither of these two weapons are used to drain him of his power. That is why he came back in WW. As for his inclusion in other games, well thats all left up to what theories one has and how they view the timeline. Personally, I see Ganondorf dying at the end of TP, losing the ToP, and sometime later being revived as Ganon. That is when he would go to the Sacred Realm for a second time to claim the Triforce, when he is sealed within it, turning it into the Dark World (events of ALttP).
Now this theory is contradicted by the widely known story that ALttP directly talks about Ganondorf (human) going into the Golden Land and being transformed into Ganon along with it whenever he made his wish. But, that leaves a big gap open from TP to ALttP as to how Ganondorf would have survived and retained his human form.