Being an insufferable elitist, on a personal level.

On a political level, putting his own ego above the survival of the revolution and deliberately violating the democratic centralism he himself had supported previously. Continuing to do so after being removed from power, then after being exiled, even to the extent of considering (or actually) collaborating with the US government to split and undermine the international communist movement.

On a theoretical level, being dogmatic and revisionist (simultaneously, somehow). He was like an inverse to Lenin’s principled scientific Marxism: elevating his own opinions as universal truths, but then retroactively adjusting them after the fact to make himself appear correct and his enemies stupid.

On a symbolic level or by association, because every trot party that came after him is at best similarly dogmatic+revisionist and insufferable, and all too often wreckers or harboring sex pests.

Some reading material off the top of my head:

Black Bolshevik has a first-hand account by Harry Haywood of Trotskyites wrecking in the Soviet Union.

Losurdo’s History and Critique provides an overview of some of the wrecking and conspiracies.

Stalin’s Wars by Roberts and Kotkin’s works on Stalin provide ample evidence contradicting most Trotsky/Trotskyite inventions, notably the misnamed “Last Testament” that supposedly proves Lenin preferred Trotsky to Stalin (if you read the letter itself, that is. The way it’s sold by trots, it somehow proves Lenin and Trotsky were besties struggling to prevent Stalin from gaining power), as well as Trotskyite claims that Stalin was somehow at fault for the Nazi invasion and/or sat by and let it happen.

But to be honest, reading Trotsky himself (after I had a decent background understanding of the Russian Revolution into WWII) is what turned me from an agnostic to a hater. Especially his letters and congress speeches, stuff he didn’t have a chance to go back and revise to paint himself in a better light.