Animals kill other animals, in competition for food, shelter, etc. (and sometimes even for sport); human beings have been pretty good by this measure, pretty successful, though not nearly so successful as some others (like bacteria). Why should we feel any particular guilt for acting according to our biology and nature?
Because rising above our natural instincts is a noble goal and being more than our animalistic instincts instead of giving in is a fight worth fighting as a species
Just a guy who doesn't want to be bored. Also