It was a coin flip

Half of him died

That's deep and all but the most likely answer is probably that Thanos is just a prick and wants to bask in his victory. Or as he said, "Watch the sun rise on a grateful universe."

Thanos was going to wipe out half of all life from the galaxy and let the stones decide the random chance of who lived and died, not half from every species. He specifically told Stark before he was going to kill him, that as a gesture of respect he was going to ensure half of humanity would still be living after he was done.