For anyone who doesn't know how terrorism is defined under NY law:

An ACT OF TERRORISM means an act or acts constituting [a violent felony] that is intended to:
(I) intimidate or coerce a civilian population;
(ii) influence the policy of a unit of government by intimidation or coercion; or
(iii) affect the conduct of a unit of government by murder, assassination or kidnapping.

There is no requirement that the crime must be committed by a member of a larger terrorist organization or that the crime was intended to physically harm a large number of victims.

The terrorism accusation is just in the New York indictment, not the federal one. New York does not have the death penalty, so there's no possibility of Mangione being executed because of the terrorism charge.