Well, thanks to Redditor u/TK-576 we found out that even if you go full pacifist and don’t kill anyone in GTA 5, you will end up killing 726 people because of the storyline. TK-576 explained the reason behind this in a post they made on r/pcgaming.
So according to the post, when you try not to kill anyone on the story, with the necessary kills for the story you will end up killing 762 people. 295 by Franklin, 258 by Trevor, and 172 by Michael. To be honest I was kind of surprised when I saw this. Because I thought that our psychopath friend Trevor would be the one with the most kills.