I - and since regretting it - rewatched the early MCU movies recently. The first time you really see Thanos mirrors the first frame, and the line immediately before he turns and smiles talks about how to invade Earth is “to court death.” Given the comic book version, I’m not sure they had the whole 50% motivation thought through yet.

I’m not sure they’ve thought it through yet, although I do like the argument elsewhere in the thread that Thanos did it to prove he was right, not to save the universe.