Even though I believe the OP to be somewhat opinion-based, I do still feel like the question has substance to it and is worth a response.
In Avengers: Endgame we see a new kind of Thanos. He is ruthless and merciless.
At that point in time Thanos had yet to acquire any of the Infinity Stones, if I'm not mistaken. He was still [at the very early stages] of his quest, and it was only due to the "shared conscious" incident with Nebula that Thanos came to learn about the locations of the stones. So, in that aspect, perhaps you could say that Thanos still had a fresh determination/fire about him (and is why he came off as ruthless/merciless).
In Avengers: Infinity War Thanos collects the Infinity Stones and in the process fights with many of MCU superheroes. But he doesn't kill any of them or even tries to kill any of them if he doesn’t have to (except for Iron Man). He seems somewhat merciful in Infinity War.
Infinity War, on the other hand, began with Thanos getting the Space Stone; and all throughout the movie things pretty much went his way, more or less, until he had collected all the stones (and of course, with each stone became that much more powerful).
Since the first stone Thanos acquired was the Power Stone, he already started off with a major advantage, and by the time he confronted the superheroes directly on Titan, he was so powerful that he maybe considered fighting them as more of a game/nuisance than a real threat. Because of this, I could very easily see him developing a kind of pity towards the superheroes.
The Thanos in Infinity War seemed to somewhat respect Tony Stark.
Yes, he did. In fact, he said it to Tony just before Doctor Strange gave up the Time Stone.
THANOS: You have my respect, Stark. When I'm done, half of humanity will still be alive. I hope they remember you.
And then lastly, if Thanos' overall goal was to wipe out half of the universe's population, I don't think he'd be too concerned with wanting to kill the superheroes specifically, or have the capacity to develop a kind of personal hatred against them... his desires were way beyond that.