And now for your trope of the day:
"The Council was divided. Mine was the deciding vote. They killed him. I was furious. I never knew myself capable of such rage. All I could see was death."— Delenn, Babylon 5
A Tragedy Of Impulsiveness is when a tragedy is caused by someone's impetuousness or irrational impatience. This trope's power comes from the fact that if characters had thought before they acted the tragedy could have been avoided.
From seeing enemies where there are none, to jumping in a swimming pool before checking for water, to messing with the Mad Scientist's machines, acting without considering consequences can cause catastrophic calamities.
This is most likely to be caused by Leeroy Jenkins, or any Hot-Blooded character in general. If the character is normally more stable, chances are this happens during a Moment of Weakness.
Related to Green-Eyed Monster, Third Act Misunderstanding, Plethora of Mistakes, What an Idiot, Cycle of Revenge, and Nice Job Breaking It, Hero. See also Fawlty Towers Plot, which is this trope's comic cousin—caused by (often impulsive) compounding lies and blowing up in the liar's face (typically causing embarrassment rather than death or suchlike).