AstroVic, you're 100% sure you did the right thing (most of us are 110% sure) - but that's realistically and intellectually sure - your heart, however, is grappling with the with the enormous emotional load of ending another human beings life. Your heart is not easily influenced by fact or reason - it seems to get hung up on things like, your innate humanity, your reverence for life, your devotion to family - things that are tough to verbalize, but are as powerful to your psyche, as fear or anger.
It is healthy to seek out others' experience and feelings - that will give you guide posts and markers, as you travel through your mind, in your seach for an acceptable, emotional resolution, to this terrible weight. You are, unfortunately, not going to resolve this, to your hearts' satisfaction, anytime soon - and unless you are an exceptional individual, you're not likely to do this by yourself. Well, actually that's not entirely true - ultimately, it is your "self" that has to provide the answers, but a professional can offer learned guidance. Do not attempt to internalize your feelings, thinking that, time by itself will eventually "heal all wounds" or you WILL become a "basketcase".
Please make sure, before you "go back out there", that you do have a SOLID handle on your feelings and that you and your heart, are of the same mind. Your profession does not allow the luxury, of a few seconds of thoughtful inquiry into the "proper" course of action, or heaven forbid, hesitation. You owe it to yourself, your loved ones, your brothers and the community you've sworn to protect - to be able to react, just as swiftly and surely, should there be a next time - as you did the last time.
You certainly don't need to hear it from me, but I'll say it anyway - you had NO OTHER CHOICE - you were not offered an alternate option. It was your turn to step into the dragons mouth - and you did - and you're still with us - share our happiness with THAT outcome and our relief that death was willing to settle for a life, other than yours.
My heartfelt wishes for your speedy return to an unburdened mind. Doug Tait.