No, it isn't right; mortal sin leads to death but it is not in and of itself death.
Ananias and Sapphira died but there is no clear indication in the passage that died without repentance although I do admit that their lie was not fully repented of.The immediate context is this:Everyone who commits sin commits lawlessness, for sin is lawlessness. You know that he was revealed to take away sins, and in him there is no sin. No one who remains in him sins; no one who sins has seen him or known him. Children, let no one deceive you. The person who acts in righteousness is righteous, just as he is righteous. Whoever sins belongs to the devil, because the devil has sinned from the beginning. Indeed, the Son of God was revealed to destroy the works of the devil. No one who is begotten by God commits sin, because God's seed remains in him; he cannot sin because he is begotten by God. In this way, the children of God and the children of the devil are made plain; no one who fails to act in righteousness belongs to God, nor anyone who does not love his brother. (1 John 3:4-10)It is not a single sin but habitual sin that characterises a child of the devil and it is not a single act of obedience but habitual obedience that characterises a child of God. That is what John is teaching. Do you agree?
Seems to me that the passage in Hebrews is about apostasy. That is why it says, "you may be sure that anyone who tramples on the Son of God, and who treats the blood of the covenant which sanctified him as if it were not holy, and who insults the Spirit of grace, will be condemned to a far severer punishment". A person who completely repudiates christianity cannot flee to some other sacrifice for forgiveness and there's no law by which such a person can be forgiven so unless and until they humble themselves and repent of apostasy, returning to Christ, asking for his mercy, and receiving grace again they can expect no forgiveness in this world or the next.