Being a former EMT myself, I would follow my own training. Call me a cold calculating SoB, but if I have to shoot you to stop you, you are now DRT.
I don't think that anyone ought to have a legal duty to rescue anyone—assuming that you haven't put a person in a position of peril that they didn't deserve to be in or otherwise voluntarily assumed a duty to provide aid. (E.g., if you push your little brother into an icy lake, you have voluntarily assumed a duty to fish him out.)
I don't think it is an injustice to let someone suffer the full consequences of threatening someone else's life. But justice and morality aren't the same thing. Justice is what one can rightly be compelled by force to do, and morality is what one ought to do.
My understanding of morality is based on the tenets of my religious faith, which instructs me to love my enemies, to pray for those who persecute me, and to act mercifully not because others somehow deserve it but because I will be blessed for showing mercy.
