Just as you can’t spend mana that you do not have, you can’t pay more life than your current life total. For example, if you have a life total of less than 5, then you cannot pay the Morph cost of a Zombie Cutthroat. This is true even if an effect prevents you from losing the game even if you have a life total of zero or less. Examples of effects that prevent you from losing the game would be Platinum Angel and Angel’s Grace.