Why would they *spoilers*

You're right. As I sat here watching the film's beginning, I found myself figuring that -- if I was a billionaire, I wouldn't fund presidential campaigns. What I would do is fund experiments like this one. (Though perhaps of orphans rather than kidnapped girls.) But not to kill off the gals. I'd be building up an ever-growing army, not whittling them down. I grew up in the love generation, not the death generation. Movie-makers continue not to think like me.

Very inefficient. Tactically stupid.

they were trained to be ASSASSINS! ruthless, emotionless assassins. even though they had all the other skills on point, if they were never able to actually kill someone, what were all those other skills good for? by killing the other girls they had lived with/bonded with for all those years, it desensitized them. at least that's how i see it justified.