I feel the same way about shoting a human or animal as you do, i had a dream years ago that i shot a person in self defence and i was depressed in the dream and depressed all day when i woke up.
HOWEVER i also had a dream that i was on an ultra exclusive island where all the worlds political figures and even terrorist hung-out regardless of current event and I tricked Bin laden into being my buddy and after i left the island he came to my house to pick me up to hang out and i killed him and his bodygaurd and i woke up happy LOL i know, weird dream that i can atribute to playing Socom III way too much about a year ago.