The purpose of dice in a roleplaying game is to give the mechanics a dash of real world probability. Without dice interjecting the elements of chance, the interactive quality of roleplaying games can become severely redundant. Although the concept of “free form” roleplaying is very attractive to many storytellers, it does require a great deal of cooperation and maturity to be successful.
Remember the neighborhood kid from your youth who, while playing spaceman and aliens, would exclaim that your laser did not shoot him because he had laser proof shields. You would exclaim that your lasers were shield-penetrating, but then of course his shields could deflect those as well. The issue is that without the limitations of the game mechanics and dice to act as a medium through which the players can interact with their world, all the suspense and conflict immediately dissipates as an imaginative cold war builds.
Granted, the Game Leader should never become a slave to his own dice. The dice are integrated into the game to allow aspects of chance and luck, but not to tell the story for you. Never allow the dice to change your story beyond your vision; instead use them to fill in the cracks of your outline. Truly the magic of a roleplaying game is in the collaboration between the Game Leader, the Players, and the Dice, accumulating into a unique experience.
The Game Leader may occasionally disregard a die roll but he should never abuse this ability or risk the fundamental integrity of his game. If the players believe that a failed roll will be disregarded by the Game Leader at any moment, than the ability of the die to create conflict has been lost. They become arbitrary as every action is easily accomplished, resulting in no challenge against the players, causing the game will begin to unravel.
The Game Leader is not god, but rather the space-time continuum. It is not his job to act vengeance upon those players who ate the last slice of pizza, but rather create a safe, fun environment where he and his friends can openly collaborate without fear of rejection. Characters will die, but it is not your job to kill them. You are not a Master, but a Leader, requiring a level of neutrality from your actions. If a character dies, it is not the end of the world. Allow the player to roll a new character and maybe even give them a few extra experience to help him catch up with the rest of the group. A death in the party can even be a good thing. If two characters have bonded and developed a friendship, then suddenly when one of them dies, the other is now flooded with emotion and motivation. Maybe he believes it is his own fault, then later must seek revenge on the actual killer. There are thousands of possible outcomes which could occur either by the Game Leaders prompt or the players own cooperative creativity.
If you do wish to avoid killing your players then there are steps that you can take to insure it will not randomly happen at the worst possible moment. The best way to do this is by giving the characters Destiny points for good deeds they have accomplished. This allows them to be proactive in saving their own life, which will maintain the conflict while prolonging their mortality.
Novice players might be quick to jump to violence in every situation. Though this is fun for them at first, it can quickly become dull, as their solution to every problem is the same. Encourage players to act how their characters would act. Inspire them to invent creative solutions instead of instantly reaching for their guns. Do not let them forget about diplomacy, bribery, bluffing, impersonation, flattery, stealth, and various other methods of trickery and non-violent problem solving. Characters who come up with creative ways to avoid danger should be rewarded with more experience than those who are always looking for a straight forward fight. Players who roleplay consistently will agree that the diversity of scenarios, characters, and approaches to conflict resolution are what make roleplaying games consistently fun for years to come. If things ever get stale, just try something new!