But you could just as easily apply that reasoning to all of the Avengers and Guardians that he encounters and spares. For example, not crushing Cap in Wakanda, letting the Guardians live at the Collector's base.

I think he tries to avoid killing outside of his "balancing" because he wants to kill a random 50%, not "everyone brave and strong enough to oppose him and then 50% of the rest." He also seems not to hate his enemies and even sometimes respect them, he just believes he's doing what must be done for the greater good.

I think he was going to kill Tony because Tony's attack had just nearly succeeded in stopping Thanos. He was a legitimate threat to the snap occurring, and that was more important than any respect or desire to reduce collateral damage.