I have no intention of seeing this film, as with a few exceptions, pretty much every Marvel Film after the first Avengers has been pretty disappointing.
To your objection though, I think the philosophical argument boils down to whether or not it is morally acceptable to kill a man before he sins, knowing he will make a sinful choice. Baby Thanos is still an innocent baby, but adult Thanos is a genocidal tyrant and the army that chooses to follow him are also guilty because they actively support him. Then the second question becomes, whether or not it's morally acceptable to take one innocent life to save so many more. It's a shame they don't at least try to explore this, but I'm not surprised. It's a mainstream comic book movie after all.
I hate these fucking movies.
If you know you probably won't like them going in, why go? I've pretty much given up on Marvel and Star Wars both, which is great because I'm pretty much done with giving Disney any of my money since they acquired Fox (I've seen a few of the films, but either only on Netflix or at a friend's house). The fact that offerings in both universes have been very subpar just makes my choice easier for me. Have you seen any movies that you have enjoyed recently? I ended up finally seeing The Sisters Brothers last night, which I have to say on the one hand wasn't what the trailer for it depicted tone wise, but on the other hand, it was more than a halfway alright movie.