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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 try to kill any of them if he doesn’t have to (except for Iron Man). He seems somewhat merciful in Infinity War.

But in Avengers: Endgame we see a new kind of Thanos. He is ruthless and merciless. The Thanos in Infinity War seemed to somewhat respect Tony Stark. But the Endgame Thanos is like, "I am going to tear down this planet to the very last atom and this time I am going to enjoy it."

So why was Endgame Thanos so different than Infinity War Thanos? What changed him?

^{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.

Originally, Thanos probably didn't care much about the Avengers other than as an obstacle to his mission of freeing the world from overpopulation. He thinks he's doing everyone a favour, even though humans and other lower life forms might not yet understand it.

However, in Endgame he learns that his future self succeeded, but even five years after the snap, the pesky humans hadn't learned to appreciate his gift but tried to undo it. That makes him reconsider his plan, to actually disassemble and rebuild the whole universe instead of just removing half the living things; and, at the same time, pisses him off enough to cherish the thought of doing exactly that.

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.

He was ruthless and merciless in Avengers: Infinity War as one can see from the opening scenes in the movie. He destroyed the entire ship Asgardians were travelling on using the power stone even after getting the tesseract. He used his army to conquer planets and kill half the population — as can be seen on Gamora's planet.

It's not that Thanos has changed but the circumstances.

In Infinity War, Thanos' objective is to collect all the Infinity stones, and he might be in a hurry to achieve that goal and he doesn't really bother to go for the kill when fighting the Avengers.

While in Endgame his objective is to stop Avengers from stopping him to get to the Infinity stones. Thanos had Nebula get the stones (which were already collected by the Avengers and saved Thanos a lot of time and also the stones were not going anywhere soon until the brown van with the quantum realm machine came up) for him and he was waiting for the battle between him and the Avengers to begin. And, he was going for the kill in the battle. He was about to kill Thor with the Stormbreaker but Cap intervenes.

So, in a sense you can say that Thanos seemed to be more ruthless in Endgame than Infinity War but under the hood it's the goals that are driving the Mad Titan.

But the Endgame Thanos is like, "I am going to tear down this planet to the very last atom and this time I am going to enjoy it."

Before telling this Thanos also says that in all his years of conquest it never got personal. But, this time it has become personal.

And maybe that is the reason he wants to tear down this planet to the very last atom and enjoy it.

It got personal for Thanos because he didn't really like the Avengers trying to undo the snap and bring the wiped ones back.

The Infinity War Thanos had a different set of experiences than Endgame Thanos.

Infinity War Thanos had been betrayed by both Nebula and Gamora. During the course of the film, he loses the rest of his servants, including Maw, which appears to move him more than he might have expected.

By torturing Nebula, he learns that Gamora had been betraying him for some time, even before the events of Guardians of the Galaxy.

The events with the Soul Stone prove that Thanos truly did love Gamora. His set of discussions with her consist of him trying (and failing) to justify himself, and his quest, to her. After failing to convince her that his quest is necessary and right, he then has to sacrifice her in its pursuit.

All of this points to a character arc where grief and loss has tempered Thanos' madness by the end of Infinity War. He's still the villain, but a different type of villain than he was when he started. Endgame Thanos had none of those experiences, so he's still the thorough monster he was at the start. That's the point of the Nebula character arc in Endgame, also - sometimes experience changes you so much that your previous self wouldn't even recognize you, and would call itself your enemy.

My opinion: he's disappointed with everyone, not just the Avengers, since in his view, the remaining half of the universe population should have thrived and grown into something great, but instead they got stuck in the past, and didn't evolve at all as he was expecting. Note: not only humans, all of the life forms.

That's why he decided to change his original plan, and tried to extinguish all the living beings in the universe in his second snap, which luckily failed.

As for enjoying killing the Avengers, that's because they made his daughter(s) betray him, thus making it personal.

Team Purple has different motivations in each movie, which explain his ruthlessness in Endgame, and his lack of bloodthirst in Infinity War.
Thanos in Infinity War wants to remove half the universe. Thanos in Endgame wants to remove ALL of the universe and rebuild it from scratch.