Because he isn't a ruthless killer. He thinks he is the good guy, saving the universe.

But he does kill Vision ruthlessly. He threatens to kill Tony. They were standing in his way of killing half of the universe, so why not just kill those that were jeopardizing his mission (who he was going to kill anyway)?

It just seemed silly.

Thanos is the "hero" in his mind trying to save the universe from overpopulation. All he had to do was complete the mission, and half of them would die with just a snap of his fingers anyway.

He did kill Loki and Heimdall when he didn't have to though.

I guess he doesn't kill people he respects. But then he respected Stark, and didn't have to kill him. I guess you could argue that he was trying to get the time stone, and as long as Tony was alive he would have continued fighting Thanos.

Maybe he sees them as leverage, peon ants that he can hold hostage if need be (and did) to retrieve all the stones.
Loki was a real threat because he can shapeshift, trick, and such. (There's even a theory that Loki is alive and poses as Bruce Banner, an explanation for no Hulking out).

@Xyberfaust said:

But he does kill Vision ruthlessly. He threatens to kill Tony. They were standing in his way of killing half of the universe, so why not just kill those that were jeopardizing his mission (who he was going to kill anyway)?

He kills Vision to get the gem. It's nothing personal against Vision.

And he was going to kill Tony but Dr.Strange made him a bargain, spare Tony in exchange for the time stone.

The rest were just insignificant nuisances standing between him and Vision. No need to kill them, he was about to complete the gauntlet and bring balance to the universe.

@Xyberfaust said:

Loki was a real threat because he can shapeshift, trick, and such. (There's even a theory that Loki is alive and poses as Bruce Banner, an explanation for no Hulking out).

Loki was killed because of his betrayal. And to torture Thor. The "Loki is Banner"-theory doesn't make any sense. We have several scenes were Banner is arguing with Hulk, trying to change but can't. Loki wouldn't do that.