Physically? Im 280lbs, 6'2, and could easily heave anyone i dont like off of a nice tall cliff.
Mentally? even more easily. Humans are animals. Were just more creative in our modes of self destruction. I see a hyena eating one of its own, and i laugh. I see a human eating one of its own. I laugh even harder. Because in all truth, were nothing but hairless dogs, who walk on two legs. And no matter how much we want to deny it, we are no different from the tree dwelling primates we call inferior creatures.
So both mentally and physically, yes. I could kill a person. Sadly, im a pacifist. ((Though i believe in violence through others for the right reasons. I wont shoot you, but if your going to cause the death of hundreds, i'll happily let someone do it for me))
And to be honest, killing a person wouldn't effect me. I used to go hunting with my dad, and taking the life of a furry forest dweller never bothered me. ((nor did i get a thrill out of it)) Probably the reason i ended up becoming a pacifist.