The Mook has failed multiple times, and Stellow has given him multiple chances to redeem himself. When it's clear that the Mook simply doesn't have what it takes, Stellow finally kills him.
While killing Mooks for their failure might lead to a loss of morale for Stellow's minions, Stellow notices that the Mook is The Neidermeyer and a General Failure. The morale of the Mook's own subordinates is already in the toilet because of their boss's incompetence, so Stellow kills him and appoints a more competent leader as his replacement...And There Was Much Rejoicing from the Mook's former subordinates.
The Mook is Lethally Stupid or Too Dumb to Live. Killing the former means that they won't mess up other competent mooks. Killing the latter is just speeding up the inevitable.
Alternatively, Stellof realizes that the Mook's talents could be used more constructively elsewhere. He reassigns the Mook to another position in the organization, and the Mook turns out to be much better at this job.
Stellof has lost the final showdown, but is alive. His mooks kill him because he lost.
Stellof Fladovski, an Evil Overlord, is informed that one of his Mooks has failed his task to capture a hero. Stellof draws out a gun and points at Mook, saying "You promise me that you won't do that again".
Alternatively, Stellof tries to kill the Mook with a gun after failing his task, but the Mook manages to narrowly escape.
Stellof carries a gun full of blanks and gives all his Mooks squibs. He feels that pretending to shoot his Mooks has wonderful stress-removing properties.
Stellof is furious at the Mook's initial failures, and warns him that if he screws up just once more, he's a dead man. However, the Mook has learned from his mistakes and starts succeeding at his assignments, saving his own skin in the process.
Mook: (grovelling and wailing piteously) "Forgive me, milord. I-I'll do better next time, I swear..." Stellof: "Silence, You Fool!. Yes, you failed. But; you were never meant to succeed. I chose you because of your incompetence. Let's hope this little exercise motivates you to better yourself. Now, get out."
Stellof: "Pffffffft... Ha! Ha ha ha! You should have seen your face when I pulled my gun! It's not even loaded! Haaa ha ha!"
Stellof is angry at the Mook's failure, but isn't about to demoralize the rest of his minions by killing him. Instead of killing the Mook, Stellof humiliates him by demoting him within the organization, which costs him money, prestige and privileges. Stellof then appoints his most successful lower-ranking henchman to take the Mook's place, with the understanding that he'll suffer the same fate as the first Mook if he screws up.
"You have failed me for the last time - you are fired!".
Stellof actually set up the mook for failure and killed him, because the mook was really The Mole and Stellof did not want anyone to know there was one in the organization.
Stellof angrily shouts "You Have Failed Me!" at the mook- then tells him to take a day off, get some rest, and try harder tomorrow.
Stellof decides that the mook's talents lie elsewhere, so he reassigns him to a position where he can be more useful.
Stellof is informed that one of his Mooks has failed his task to capture a hero. Stellof draws out a gun and points at Mook, saying "You promise me that you won't do that again," the Mook says "I promise" but Stellof still shoots him.
Stellof initially looks like he's about to kill the Mook for failing him, but proceeds to kill one of the Mook's immediate subordinates. The subordinate Mook was the one actually to blame, however.
But the mook's car blows up as soon as he turns the key.
Parodied: Stellof goes to a restaurant where the waiter gives him the wrong order, so Stellof tells him "You have failed me" and kills him.
Zig Zagged: Stellof tells the mook he has failed and draw his gun. He tells the mook not to let it happen again, turns to the man who hires mooks and shoots him for hiring incompetent staff.
Averted: Stellof's Mook has failed his task but Stellof just berates him without physical threats.
Stellof is not overeager about executing his mooks, reserving it for the more egregious failures. Combined with the fact that he's actually pretty nice when his men are competent, this means his soldiers are highly motivated to succeed.
Discussed: "We can't go back without the MacGuffin! He'll go 'You have failed me for the last time!' and throw us in the shark tank!"
Conversed: "Must you kill every one of your men who screws up on a job, Stellof?"
Stellof becomes powerless since he has killed the best of his minions and others are turning against him and performing Heel Face Turns in fear of getting killed by him.
The remainder of Stellof's (less competent) officers and minions begin covering up their mistakes and making false reports out of fear of suffering the same fate. This comes back to bite Stellof in the ass when he overestimates his position...
Stellof ends up doing this perpetually, lowering morale and costing him potential recruits, since his minions are never given the chance to learn from their mistakes.
Reconstructed: Stellof only executes his minions for mistakes that were the result of gross incompetence, not of simple misfortune.