Leaders and factions are not a problem in a system that does not allow them to easily hoard power. Which authoritarian socialism isn't, of course, but that's not the only kind.
If you're wondering why we don't see the other kind much - well, the socialist movements that won back in early 20th century were generally the most ruthless ones, whereas the more democratically minded movements (e.g. Luxembourgism or various anarchist groups) were crushed. So you had a kind of artificial selection for brutality to begin with - but then countries controlled by those winners would also subsidize similar movements elsewhere, and of course they demanded ideological conformance in return. This is largely why stuff like Marxism-Leninism and Maoism is still so pervasive on the left.
If you're wondering why we don't see the other kind much - well, the socialist movements that won back in early 20th century were generally the most ruthless ones, whereas the more democratically minded movements (e.g. Luxembourgism or various anarchist groups) were crushed. So you had a kind of artificial selection for brutality to begin with - but then countries controlled by those winners would also subsidize similar movements elsewhere, and of course they demanded ideological conformance in return. This is largely why stuff like Marxism-Leninism and Maoism is still so pervasive on the left.