Imagine going into Infinity War for the first time already knowing that Thanos succeeds in snapping half of all existence away. Will the movie still be good? Of course it will. It's one of the best films of its genre, if not the best ever made.
Still, if you know about its major events in advance, you will never, ever experience that visceral, emotional response of witnessing it for the very first time as the creators intended. Is the movie ruined if you have the ending spoiled? Maybe not. It's still a great movie on repeat viewings...but a huge portion of the payoff is missing if there's no suspense in those crucial moments.