It is not to prevent the birth of Naruto that he wants to kill Minato but to prevent the massacre of thousands of Iwa shinobi at the hands of Minato.
I am sure this Anbu already sent an emergency help request to Konoha, I need to kill him and Minato quickly and run away bringing Nawaki and Fugaku as hostages.
Hard to reincarnate into world of naruto as Iwa shinobi