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You've kind of got a constellation of tropes intersecting here:
Badass on Paper and Self-Fulfilling Prophecy for the technically accurate reputation that wins the fight for them
The Dreaded and Horrifying the Horror for the reputation itself being enough to scare people into screwing up- they delve into Terror Hero if they've figured out that they can do this on purpose.
They're Lethally Stupid if they kill people via What Does This Button Do? and Unwanted Assistance-type situations. It's very common for this to be the modus operandi of the Invincible Incompetent Idiot Hero.
Edited by Scorpion451

Is there a trope for when a character/thing is considered very dangerous, but is actually harmless and/or unaware, and people are just getting themselves killed or wonded by their own idiocy - often by being so scared and cautious that they end up getting hurt.
For instance, the Fake Ultimate Hero is rumored to have killed a thousand people. He threatens a bad guy, the bad guy gets scared, runs off, trips and falls off a cliff. Then in the next episode, the hero is rumored to have killed a thousand and one people...
Edited by Mac_R