'for he was mine to kill, not yours, had I chosen'

The protagonist is a prince of Amber, the only real world, has amnesia, but tries to conceal the fact. While he and his brother are driving to Amber through several Shadows?the parallel worlds of Amber. They almost hit the truck, the driver of the truck gets off, and curs at the protagonist.

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Random raised the pistol and took careful aim at the man's back, and I managed to knock his arm to the side just as he pulled the trigger. It scored the pavement and ricocheted away.
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"I can take care of my own honor," I told him, and something cold and powerful suddenly gripped me and answered, "for he was mine to kill, not yours, had I chosen," and a sense of outrage filled me.

I think the underlined clause should be "for he would have been mine, not yours, had I chosen to kill."
So I was wondering how "to kill" could be shifted before ", not yours," and why "was" was used, not "would have been," in the conditional 3.
Thank you in advance for your help.