So I Might Be Insane, But-
At first, Prime wasn’t trying to kill anyone. Not necessarily.
He defended himself from their attacks. He threw the brothers away. And he took their powers. All to keep them off the platform.
He didn’t seem to care about the battle at all. His first priority was to clear the way for his Brother and Sister to follow him through the portal. That’s all he cared about. His next step, to bring their mothership, their home, their suits, and anything else, with them.
Prime ignored the battle that Sister rages entirely. He followed Brother over to the portal to watch over him. He wasn’t helping Brother, and yet, he clearly had plenty of faith in Sister to man the battle. Did he not trust Brother to do it on his own?
Prime merely stood and he watched. Why?
I didn’t think much of it until the following scenes.
When Leo explodes the portal and the Brother on it.
Interrupting the altercations of the portal is clearly a very deadly endeavor, and Leo basically just blew Brother up. I have to wonder if Prime knew that was a possibility. Brother is straight unconscious, blown away by the force of the explosion. Prime has no hesitations in catching him, even before he’s certain that he has the footing to steady the both of them.
Once the chaos stops, he gently sets Brother down beside him and narrows his eyes in a way that promises to return the pain. Sister’s immediate reaction is to shriek and look to Prime. She doesn’t pounce or shout or try to rip the key from Leo’s hands. She looks over to her brothers, checking on the one who is unconscious and pained.
It’s Prime, with a silent fury that promises all bloodshed and no mercy, that mans the attack on the blue masked creature who showed up when everyone else left, interrupted their plans for freedom and corruption, and put his Brother and Sister in peril.
Now he finally chooses to fight, rather than to defend, and to put an end to the miserable little nuisance who’s more trouble remaining alive than it would be to kill him like he tried to murder his Brother.