Do not be afraid to alter outcome resultes to enhance the suspense or heighten the feeling of the experience. You should NEVER set out to kill PCs. All things being equal, you should NEVER alter a check that directly results in killing a PC. The GM is NOT the adversary of the players – the GM is the teller of tales, there to ensure the action is balanced and fun for everyone in the group (including themselves).
Single Check Outcomes are Lethal: A checked outcome may also be assessed using Advantage or Disadvantage. This speeds up combat and emphasizes skill over luck, and is to be used unless noted in materials. It also rewards tactical planning and forethought in committing to combat because it is more lethal.