I'm guessing that when he saw her get hit, rather than feeling good about it like he would've if a cat got hit or if someone else had gotten stabbed, he instead felt nothing but bad emotions. Which would've surprised him.
the butterfly effect is much more adverse for her because makoto wasn't ready for mai's death so he goes completely beyond bonkers since mai makes his life somewhat bearable with the impromptu pranks so when he saw the possibility of her death, he can't let that happen unless he wants his life to go back to being monochrome.