It’s not murder because the OP expressly presumed self defense (“…if I’m acting in self-defense?”). If you have a citation for a statute (ETA in the criminal context) that confers a duty to render aid or summon authorities after defending oneself, I’d be very interested to read it.
“Immediate” matters because the OP inquired whether one could “let them die before calling the police.” And, as noted in my prior posts, because I’m aware of no law that makes it a crime to fail to immediately report a self-defense incident but there are laws that will subsequently require a death/corpse report given the facts in the OP’s hypothetical.
It’s fine to cite the definition of actus reus from a law school textbook. But textbooks do not codify crimes. In order for there to be a crime, there must be a statute that makes that act a crime. Look up “fair notice” in the same law school textbook. Or read what the Supreme Court has said about it:
"A fundamental principle in our legal system is that laws which regulate persons or entities must give fair notice of conduct that is forbidden or required… Living under a rule of law entails various suppositions, one of which is that all persons are entitled to be informed as to what the State commands or forbids.” FCC v. Fox (citing a host of other Supreme Court cases on fair notice and due process).