Yes, any rationality based on simple ethics and morals will tell you to save the person. The only type of person that would not save the person is probably evil and only considers themselves before others. Try to imagine yourself in the position of this person and someone could kill or save you; the chances are that you would rather have them help you rather than kill you. But in the end it is up to the person pressing the button, and in my personal opinion it is irrational not to help.
"Try to imagine yourself in the position of this person and someone could kill or save you"
thats empathy, the question is, if one doesnt have empathy then would it be irrational for him to not save him? if lack of empathy prevents you from helping others then could it be that helping others is not the product of rationality but of empathy?