The left's main vulnerability is splitting and infighting. We are terrible at it. This is known.
To build revolutionary change, a revolutionary party is needed. It must do its work and carry on its work. In the military, they taught me that the number one priority is preserving your ability to fight. It's ok if you have setbacks, lose equipment or men or tactical position or advantage; but it's a catastrophe if you lose the capacity to fight.
Leftist infighting is what robs the movement of its ability to fight on. It kills the movements. (The Buddhists know this, and consider 'creating a schism in the sangha' the supreme sin. That's a tangent, but the Buddhists have maintained an ideological movement for 26 centuries so they're doing something right.)
We need to mock people who shame other leftists for impurity, who insult them for making mistakes. Liu Shaoqi said: "The attitude of some Party members towards these shortcomings and mistakes and towards those comrades who have incorrect ideas is one of “bitter hatred and gall”. They lightly sever all relations with comrades who have committed some mistake and whom they attempt to expel from the Party outright."
It is ok for comrades to make mistakes in action, and to have mistaken beliefs. That's the completely normal, inevitable part of having a movement. In fact, that's all a movement is: listening to people's beliefs, listening to their wrong beliefs, correcting them, unifying the thought of the party, unity makes strength. It is not ok to have incorrect ideas of “bitter hatred and gall”, and lightly sever all relations with comrades who have committed some mistake.
MAO MADE 30% MISTAKES, INCLUDING CAUSING THE BIGGEST FAMINE IN HUMAN HISTORY!!! If you can forgive that, you can forgive someone who made an off-colour tweet when they were a teenager.
People who try to cancel comrades for stuff they did/said in the distant past, or for non-core differences of opinion, are dong the imperialists work for them: dismantling the socialist movement.
PS: the aim is always to have a cohesive, united, strong movement with the correct views. Anything that serves the aim is good. Anything that wrecks the aim is bad.
A key motivator to splits is the desire for power, the need to be The One Person Calling All The Shots. People crave power, amd when there is a ladder of command, they wish to climb it.
Any organization that distributes its roles of power, and eliminates the concept of overall stratification, is going to make itself less prone to splits.
Having multiple ladders of power to climb, separate domains of expertise that are not ranked against each other, such as theory and analysis, recruitment, propaganda, facilitation and mediation, will allow people to fulfil their human nature* by pursuing distinction, while not stretching anyone too thin, nor putting all the party's eggs in one basket, nor limiting how many people can wield a personal influence on the group as a whole.
*Yes I said those words. Pursuing distinction and social value, broadly, is one of the few things that can truly be said to be universal human nature rather than frequent human tendency.