I think just about anybody could commit murder if they absolutely had to. The fight or flight response in humans that some people have mentioned up above is a perfect example. Survival is a natural instinct and I think most people would do whatever it takes to survive and/or remove themselves from a life-threatening situation, including taking somebody else's life.
I also believe that being the emotional creatures that humans are, it is possible to have those emotions blind us sometimes into doing things that we did not know we were capable of before, and would not have done if we had just taken a moment to think things through. In summary, a sudden provoked response, where an immediate emotional reaction comes into play.
The only examples I can think of at the moment are fairly morbid, but lets just say that a kid is abused heavily by his father everyday for years. He is physically beaten, and emotionally demeaned and belittled until one day he just snaps and while his father is thrashing him around, he grabs a knife off the kitchen counter and stabs his father to death. It is not a pretty picture to imagine, but I think alot of times the murders that people commit are just sudden impulsive reactions like this, while in the throes of great rage or passion, and are not carefully calculated and thought through beforehand.