Ok look I know you like to think that your going to be noble but, you wouldn't take the time to trip them you'd be running your butt off ok. Second, you cant run forever so if you did take the time to trip your friend you'd probably do it to keep them off of you for a while. Third In a lack of food situation what if your friend tried to kill you because you were low on food (5 days with 2 people, 12 or so with 1) this goes with the question If your friend was Infected and going to turn would you waste him....
ive been playing zombie games sence i was 5 so i know the best way to kill a zombie is a blow to the head, exept me and my best friend woud stay and fight, not run, therefore i have no need to trip him