>>100231023Real world answer is that the cinematic universe may have an outline, but it's been revised and tweaked constantly for 10 years. Dozens of creators have had their hand in the stew. People have gotten fired, been replaced, changed plans, etc. This was originally going to be a Joss Whedon joint. Imagine that.
In universe, a lot of what we hear about Thanos offscreen is more or less hearsay. People who escaped his slaughter fear and despise him, and that colors the description of his actions that we hear. Conversely, a lot of what we get in IW is from Thanos' perspective, who compartmentalizes and rationalizes his actions.
Thanos has a benevolent endgoal, but he's an extreme pragmatist. Letting Ronan exterminate the Kree if it gets him the Power stone (and keeps Ronan in line) is fine because what is one species in the grand scheme of the entire universe?
With the Dwarves, letting them all live when they know how to make weapons strong enough to stop him was too high a risk. Letting the leader live seems like a cruel form of justice (for betraying the Asgardians and giving in, maybe), and also serves as an act of self-sabotage on his part. We'll hopefully get more of Thanos' self-sabotaging nature.
By the end of the movie, Thanos is basically all powerful and getting angry at the gnats in his way is beneath him. He's already won even before Strange gives up the time stone, and maybe he's still hurting from Gamora's death. Half of these people will be dead in just a few moments anyway.
That's how I try to reconcile it, at least.
