Some Muʿtazilī discussions asked whether a victim would have lived longer had the killing not occurred. That formulation treats the killer’s act as a cause that ended a possible further life.
The reply distinguishes the term that actually occurs from a counterfactual assumption. “Had the person not been killed” is a supposition about an unrealized case; it does not prove that the actual divinely appointed term was some different moment.
The schools analyze such conditions differently, but the key issue is that death must not be treated as an event independently escaping divine knowledge and power.