So, there's this thing that every DM needs to know. Okay now, I know I said you need not follow what I tell you, but at least consider this thing I'm about to throw at you in this post.
I've heard countless times about Dungeon Masters who, for some reason, got angry at players, and simply killed them ingame. Well, to me, that's just stupid. Being a DM doesn't mean you're playing against the fellowship. In fact, a DM is a player as well, it's just that his role in the game is a bit different.
First of all, before starting a campaign, the DM should point out to his players that he is also playing the game for fun, just as they do. Then, players should be told not to question DM's decisions, even if they happen to go against the rules, because a DM wouldn't do stupid or irrational things for no reason.
Be friendly to your players, give them a good campaign with lots of opportunities and fun, try to make the game smooth, and if they happen to mess up big time, always give them a way out of the trouble they've put themselves in. Having your player's characters die is a bad thing, and intentionally killing them is even worse. Of course, I'm not saying you should defend them at all costs, if they really deserve to get killed, there's nothing you can do, but you should always make sure they know what dangers lie ahead.
Here's a little example:
Player wishes to try and jump from one cliff to another. The gap is pretty wide, and the height is frightening. The fall would surely kill him.
- DM: The cliff you're trying to jump on is pretty far, the difficulty class is 25, your jump skill is 7, so, a roll of 12 or less instantly results in your death, while on the rolls of 13-17 you still got a chance to grab to the side of the cliff and maybe pull yourself up. Do you still wish to proceed ?
- Player: I do.
- DM: Do you understand that you got more than 50% chance to get your character killed if you try to jump to that cliff ?
- Player: I do, and I still want to take my chances.
- DM: Okay, I warned you. Roll the dice.
Now, on the rolls of 13-17, a good DM would find a way for that character to save his life, while a bad one would make sure the character falls and dies.
Thanks for reading and have fun playing! :)