You're focusing on negative possibilities as being appoximately a certainty. Once the others appeared in the target x-section, Beck should have backed off. It wasn't his call to take, unless he was certain of doing no harm. In any event he must accept responsibility for what damage he caused.
"Logan was about to enter the vehicle of an accomplice get-away driver."
Stretch. He's not even 50yds away. He can see what's going down. The woman probably screamed.
The motive here is payback. He wasn't doing anyone any favors by shooting at that van.
No doubt Beck was as well. And frankly, I think a reasonable person in his situation would have done likewise.
The vast majority of people have an aversion to killing others without provocation. Even the vast majority of robbers, who have no qualm about threatening others with death, have an aversion to actually killing their victims. Logan clearly demonstrated that he had none. Indeed, Logan had demonstrated that he had no qualms about killing even an entirely-cooperative victim; he was thus far more dangerous than a typical robber, as Beck was keenly aware.
Although it is quite plausible that, had it not been for Beck's earlier shots, Logan would have chosen someone else to be his next victim, I can't see that he's in any way to blame for his chosing Ms. Denmark. After all, if I inadvisedly take a 'hit' at a blackjack table and get a card that would have benefitted the next player, am I at fault for that person losing?
As another hypothetical, suppose that you are standing outside a restaurant and see that someone has lined up fifty people against the wall and is shooting one every few seconds. The shooter is between you and the innocent people. What do you do?
If you shoot at the murderer, you'll risk hitting one of the innocent people lined up against the wall. But every few seconds you tarry another of his victims will get shot. Do you wait until you can get a clear shot without anyone else in front of or behind the shooter, or do you say a quick prayer, take aim, and try for the best?
The fact of the matter is that while the notion of "first, do no harm" is often a good principle to live by, one who obeyed it slavishly would often be paralyzed. Like it or not, doing good sometimes requires risking doing harm because risk of harm is not always avoidable.