Personally, while I understand why he did it, I still think it was the wrong decision.

If you look at it from a black and white perspective and in terms of the bigger picture, Joel absolutely DIDN'T make the right decision, especially considering that it's what Ellie herself wanted at that time. He basically said "Fuck you" to humanity so that he himself could still be happy and have Ellie, which, on the surface, looks pretty damn selfish.
Especially since you think about people like Henry and Sam from earlier in the game who have been forced to go through some horrible shit of their own and how their story ended, and you multiply that out to the entire human race. Then, once you do that, and you go back to Joel, and it's like, "Dude, you think you're the only one who had to suffer? You think you're the only one who lost his kid or someone they really cared deeply about throughout this whole fiasco? You think that this sucks just for you? Look at Henry, Sam, and Tess. You think they didn't go through shit of their own? Your own brother? His struggles and fighting could have finally been over and he could've had his family in peace if you hadn't flushed it down the drain."

He made his decision from a place of grief, sorrow, and pain. But considering what he had to go through, what other place could he have made it from? The fact is, it's not as black and white as I seemed to have described, in fact, this situation is as gray is it could get.
Do you want to save all of humanity at the expense of you never being happy again? That would lead to an even more depressing, and even tragic, path for Joel had he done that. It's a really fucking tough choice to make when you're in that position, and at that time, he thought with his heart instead of his brain. He also could have thought about Henry and Sam and thought that he didn't want to end up like Henry.
So while I don't think it was the right decision from the bigger perspective, I don't condone him for it because, I sure as hell wouldn't want to be in his position. Who would?
That's one thing that I'm hoping gets addressed in the second one, especially what Ellie must think of it.

He made the right choice. Think about it. If Elle did have the cure and they pulled it out of her humanity would still be doomed, cause the real enemy all along was other humans. The zombies are pretty much under control it's the humans that know how to shoot a gun, the war starters, the cannibals, the invaders, and the kidnappers that are the real bad guys.
Joel realized that humanity can't be saved because society have already collapse and no cure is bringing it back, so why sacrifice Elle?

Not to mention the fire flies would have likely used the cure to subjecgate the rest of humanity.

I think he did the right thing. Some people think of Joel as a bad guy for supposedly dooming mankind but at the same time, I'm not sure Joel is supposed to be totally good. He can be pretty brutal at times but he's a product of the equally horrible or worse world around him.

Joel's a more interesting character that way.

I dont think anyone sees him as a bad guy.

After my first playthrough i would have said no he made the wrong decision, however on my second playthrough i found the various recordings in the Firefly facility in Salt Lake how many that were immune did the fireflies kill.

The fact these people had a natural immunity they were likely the last hope for humanity not for creating a vaccine but by passing on their immunity to their children.

Well Joel had already decided to rescue Ellie by the time you find those recordings.

He didn't make the "right" decision from an altruistic, worldly sense. Instead, he made the natural decision. If Joel decided to let Ellie die, or if the decision was left to the player, the ending would have been completely contrary to Joel's character.

Would have done the same,at having a second chance having a daughter.
Any time of any day i wouldn't even hesitate tbh.

No, it was the selfish choice to make, however in his position I would've done the same thing he did. We are only human after all, not to say he isn't a bad guy, just he made a selfish choice to have a little more time with his daughter-like companion, then to live in a world without her.

Potentially dooming all of mankind might convince some otherwise.