In the matter of recent events, I have concluded that people die when they are killed.
Naturally, you will be saying, "Yeah, obviously."
To that, I call you a fool.
The reasons are simple. It is not just that they die when they are killed but in fact that when Saitama punches, their soul extinguishes. The body and soul both stop and crumble. There is nothing left. Do you ever wonder the reason why Saitama is so powerful? It is because his punches rebound around spacetime and through the Soulspace.
The Soulspace is punched, and whatever entity he punched has their soul removed bit by bit depending on Saitama's current output. Each attack Saitama brings will lower the soul of an enemy.
Now you might ask, "But Garou got stronger!"
Yes, at the cost of his sanity. When Garou was getting stronger fighting Saitama, Garou was taking bits of the Soulspace to help him. This restored his soul and mainly his humanity.
Now, moving onto the reason why Sonic's soul was never extinguished: The Soulspace is only punched when Saitama punches a monster. Their souls are reflected onto reality and only Saitama can punch them.
Thus, people die when they are killed.
Another point you might have is that Saitama is monstrously strong. There's a reason for this too.
Saitama is secretly the CREATOR of the Soulspace. Saitama is the God above Gods.
Never underestimate this man.
Before Saitama killed the Crablante loser, he was just a nobody. Then the Soulspace God died and his soul, too powerful to be in the Soulspace, entered a body of someone whose soul was never inside the Soulspace to begin with. This leads to Saitama, average Joe with No Soul.
There was a reason why Saitama was such a blank slate, he never had a soul. Just a sense of morals. Then the God's soul merged with him and he became who he is.
One Punch Man.
You are welcome.