The Bolsheviks spent more time and effort fighting other socialists than they did their ideological opposites. The reason is that your total opposites can be demonized, slandered and attacked with impunity, while factions that share some of your values may appeal to your followers are possible allies and amenable to compromise and negotiation. They might even siphon off some of your own people by offering them more palatable alternatives than all-or-nothing. This threatens party discipline and control.

When Castro took over in Cuba he first locked up the Communists, then went after other revolutionary groups not under his direct command, regardless of their position on the political spectrum. The Nazis under Hitler liquidated the leadership of the SA when their followers became a threat. Ideologues only differ in degree, not in kind.