It may not necessarily mean it's an insta kill, but more like once he decided that the wants to defeat Zeno, fate (which spans across all space and time) predetermines that outcome. We don't know for sure how Zeno will be defeated, but we are sure that it will happen, somehow (fate can make the unlikely possible, as it is described as being able to grant wishes in the game). Whether a simple punch will defeat Zeno, or Shadow making Zeno feel extreme guilt for killing him (resulting in Zeno ending his own life) are two of the many possibilities that could lead to Zeno's defeat. It's just that the punch throw would be faster than the whole guilt process, for example. But an outcome chosen by fate out of the many possible ones becomes predetermined by fate to happen, having a 100% possibility to go through, and cannot be avoided no matter how much Zeno would try to prevent it from happening.