Police knew by the time they arrived on the scene that Blake had a warrant for his arrest on a sexual violence charge, was known to carry a gun, and had a history of assaulting police. Blake resisted being taken into custody. Tasers were deployed but didn’t work, as happens quite often (you need two good contacts with the subject’s skin to make a circuit.) Blake can be seen with a Karambit-shaped knife in his hand, one so deadly it has special restrictions under various laws. The officers repeatedly tall him to drop the knife, and when he refuses draw their guns.
Blake attempts to get into a vehicle, the keys to which he stole from the complainant. He is thus about to be armed with a knife, a 3000 pound vehicle, and with whatever weapon may be inside. There are children in the back seat who are liable to become shields or hostages.
At this point Blake poses a deadly threat to the officers and other people. As per their training, the cops fire at the center mass of his body until it is clear he poses no immediate threat. They had to ensure no rounds endangered the children in the back seat. This meant they fired into his back. If the intent was to kill him, they would have aimed for his head.
Perhaps one may arrive in retrospect at better ways for the police to handle the situation, but in the circumstances, they did what they had to do. The fault is entirely with Jacob Blake.