Thanos, unlike many of the other marvel villains, doesn't -think- he is evil. He sees himself as the "ultimate good guy", doing what has to be done, albeit "unpleasant", to "save" the universe.
He recognizes other pure, idealistic "good guys" like Cap, and Thor - he is not going to relish defeating or killing them. He isn't killing off half the universe for personal glory, but to make life better for the remainder. After he does so, he doesn't set himself up as a god over the remainder - he retires away to a small cabin, feeling though his huge "sacrifice" has been done, and he has fulfilled his purpose.
Thanos: ..It was beautiful. Titan was like most planets: too many mouths, not enough to go around. And when we faced extinction,
I offered a solution.
Strange: Genocide.
Thanos: At random. Dispassionate,
fair. Rich and poor alike. They called me a madman. And
what I predicted came to pass.
Thanos: With all six stones, I could simply snap my fingers and they would all cease to exist. You know what I call that?
Mercy.
Strange: Then what?
Thanos: I finally rest. Watch the sun rise on a
grateful universe. The hardest choices require the
strongest wills.
Thanos: "I
saved you."
Gamora: "No, no, we were happy on
my home planet."
Thanos: "Going to bed hungry. Scrounging for scraps. Your planet was on the brink of collapse.
I was the one who stopped that. You know what’s happened since then? The children born have known nothing but full bellies and clear skies.
It’s a paradise."
Thanos: Little one, it’s a simple calculus. This universe is finite, its resources, finite… if life is left unchecked, life will cease to exist. It needs correcting."
Gamora:"YOU DON’T KNOW THAT!"
Thanos: "
I’m the only one who knows that. At least
I’m the only one who has the will to act on it.
He sees himself as a savior, who failed to save his own people, and is now redeeming
himself, by
saving half the universe.
I believe this is the thematic reason he doesn't appear to fight "full on" and kill everyone who puts up a fight, necessary or not. I think he identifies with (or pities them) in a way. He wants to save people. He still needs to consider himself the ultimate "good guy"