Most disregard the more subtle implications of the butterfly effect, especially in a temporal sense. Sure, perhaps the flap of a butterfly's wings on one continent could lead to a cascade of events that initiate a maelstrom on another, but a more cosmically intriguing and horrifying implication is the ripples of a subtle changes in the past propagating into the future: the foundation of the theory of the multiverse.

For instance, say you have a small pod of turtles who happen to get doused by a tumbling vial of experimental mutagen, just as what happens in an infinite number of worlds in the multiverse, except this time, the vial tumbles slightly eschew, nearly imperceptible by a casual observer. Sure the vial shatters much as is usually does across worlds, but this time, one particular turtle gets a few extra drops of the mutational elixir.

Though a seemingly meaningless difference, said turtle, typically destined for a virtuous leadership role in his soon to be mutant family, has his DNA altered ever so slightly, prompting his growth plates to propel the turtle to a larger/taller stature than even his second tallest brother even at an early age. Said turtle's internal sense of control typically parlayed into a compassionate self-less leadership role is hijacked as now said turtle is able to exert their control physically. Once the young Leo learned he could get what he wanted through brute force, he did so, attempting to shape his isolated world by pushing his brother's around, at first incidentally but with repeated success more and more deliberately. It reinforced a sense of superiority in him that continued to growth and manifest as narcissism, egotism, and psychopathy: a stew of emotions that shaped him as the family bully.

While normally Leo was a leader by example in other worlds, in this particular universe, he lead by fear and control that he made sure to routinely exercise to keep his brother's in line and subject to his whims. Here we see Donnie once again futily attempting to retrieve his coveted textbooks while Leo lords it over him, standing nearly 7' tall over his already tall purple banded brother. Donnie is going to have to try harder than that if he's going to get his book back, either that or wait for Leo to grow bored and find one of his other brother's to torment...

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An awesome commission from MutanTAIL helping to bring to life this AU I've had in my head of BULLY!LEO.