A couple of things from my days as a P&P GM...
1) Dead characters don't tell good stories.
Pretty straight forward and probably the most important thing, as a GM you're telling a story. If everybody dies on page one the story doesn't get told.
2) A GM should never kill a character, killing a character should be down to the player.
The character that walks into the unmarked mine field is killed by the GM. The character that walks into the marked mine field is killed by the player. No 'marking' the mine field doesn't have to involve a sign post that says "Mine field", half a dead cow in the middle of a field does the same job, also if the players encounter other traps on the run up to the big one it should teach them to be searching for such things.
3) Death should always mean something.
Again the player who's character steps on a mine they have no way of knowing is there is likely to be... annoyed... The player who gets to make a last stand against overwhelming odds while their team mates escape will feel... heroic. There's nothing wrong with a character dying, just don't make it cheap.
Don't get to attached to dice rolls. If the players need to do something to progress the story then let them do it, don't let a bad dice roll kill them off. That said there can still be a cost, the rules may say that the player climbing the cliff who fails a strength test should fall to their death but there's no reason why that character couldn't ditch their backpack (Food, ammo, primary weapon) to be able to hold on. A character who falls to their death does nothing to enhance the story, one who suddenly needs to find food and water does.
As a final thing: Run the game you want to run. A GM who doesn't enjoy the game they are running is normaly a bad GM. Chances are you'll find players willing to give the game a go no matter what topic you pick.