It wasn't the Marxists betraying the revolution, though. There's genuinely no need for anarchists and Marxists to kill each other, the idea that Marxists always "betray" anarchists comes from the examples of some anarchists choosing to take up arms against socialist states and being killed by the Marxists. There are numerous examples of the opposite happening, and many of anarchists joining the bolsheviks and other communist parties out of sheer practicality. The subsection of anarchists that were killed by Marxists weren't killed "for not being reactionary enough," but for quite literally being reactionary and choosing to attack socialist states.