Por que Krennic tem esses personagens mortos?

A partir da novelização, parece bastante claro que Krennic está tentando desmoralizar Galen; Galen obviamente não quer que os outros engenheiros paguem por suas ações, então Krennic os matou para demonstrar seu controle e as conseqüências de cruzá-lo (grifo meu):

Galen took action at last.

He dashed between Krennic and the engineers, spun about and nearly slipped on the wet platform. "Stop," he cried, again and again, spreading his arms as if he could block the troopers' shots. "Krennic, stop. It was me. It was me. They have nothing to do with it."

Krennic looked into the face of the man he'd befriended long ago. He waited.

"Spare them," Galen said. Drenched and tired and wild-eyed, he looked like a man whose genius had deserted him.

Krennic crooked a finger at Galen. As if reluctant, the begging man stepped back toward Krennic.

"Fire," Krennic spat.

He didn’t watch the crimson bolts flare from his troopers’ rifles, didn’t bother glancing at the bodies of engineers tumbling to the ground and sizzling in the rain. His eyes were on Galen, and he saw the explosion of shock and fury in the scientist's face — saw him try to hide it the next instant behind a mask of iron.

But they were long past hiding things, and Galen should have known better. Krennic swung his fist in a tight arc and felt the back of his hand strike Galen’s cheek and chin. Galen staggered and dropped to his knees.

^{Rogue One: A Star Wars Story - Official Novelization Chapter 11}

Krennic está de mau humor. Ele acabou de ser humilhado por Tarkin e teme que ele tenha seu brinquedo tirado dele. Inicialmente, ele suspeitou que um dos engenheiros era um traidor do Império (daí porque ele os colocava na frente do pelotão de fuzilamento), mas no final ele os matou simplesmente para desabafar sua frustração.

Krennic let them stand there in the rain for a bit longer than necessary, just to show them who was in charge.
There was no Tarkin present, no Emperor, no Darth Vader. When it came to this facility, he ruled.

...

If that didn’t get them to crack, Krennic believed nothing would. Then he’d be forced to have them all executed. Until now, he might have hesitated to give such an order for fear of destroying a team he very much needed, but with the Death Star operational—and Tarkin having stolen it from him anyway—he didn’t see the harm.

...

Krennic looked to the engineers again. They were awash with relief that Galen would take the fall for them. They were too surprised—too unsure about what was happening—to thank him for his sacrifice.

Krennic stared at them, despising them all. He said one word. “Fire.”

_{^{Rogue One: A Star Wars Story – A Junior Novel}}

A execução que vemos não chega nem perto de todos os envolvidos em design e produção. Se eles precisam de mais trabalho, eles têm outros engenheiros para fazer isso.

Para Krennic, a pequena perda da vida de engenheiro vale a pena.