There are two meanings of ‘sectarian’ that are distinguished by exclusive and hegemonic behaviour.
I tend to think of them as the ‘Protestant’ and ‘Roman Catholic’ models of sectarian
‘Protestant’ sectarian is exclusive. There is a narrow-minded, bigoted adherence to particular doctrines. This is protected by the practice of expelling or excommunicating those who do not agree. It tend to be non-political in the sense that ‘Protestant’ sectarians are content to maintain control of their isolated group and are reconciled to the majority not conforming to their view. Historically (e.g. 17th, 18th Centuries), this has led to the practice of toleration (i.e. putting up with views you abhor – on this definition Stalinists are the most tolerant people in the whole world). The British ‘Left’ is a good example of ‘Protestant’ sectarian: moral probity always trumps politics (e.g. “Bomb and invade who you like, I won’t stop you; just don’t do it in my name”.)
‘Roman Catholic’ sectarian is hegemonic and seeks to dominate and ultimately absorb dissident views. ‘RC’ sectarianism is highly political in that it seeks to control the state as a means to imposing its views on the whole population e.g. Ireland.
Obviously, the labels are a bit arbitrary. Many real Catholics are ‘Protestant’ sectarians and vice versa.
Lenin, like Stalin, was a mixture of the two. Obviously, Lenin was keen to defend the scientific, Marxist, point of view to the point of splitting organisations. Conversely, he never let morality get in the way of politics. On the third hand he was tolerant of those with different views as long as they didn’t cause trouble.
This last bit is too simplistic but it will have to do for now.
George, I think you have split into ‘ideal types’ a pattern that is characteristic of RCs and Protestants. It may be called a universal of exclusion: if you can’t absorb them, crush them. Or rather, RCs operate with a single, exclusive universal (‘catholic’), while Protestants have multiple universes. That can lead to mutual intolerance, or it can lead to recognition of these different universes. The extraordinary breadth encouraged within the Netherlands during its economic peak in the 17th-18th centuries is a case in point. This was not a commercial exception to narrow Calvinism, but deeply implicit within Calvinism.
As for Lenin, at his worst, he lost sight of the deeper dialectic of sharp and vigorous debate and let it slide into bitter factional strife. The amusing point here is that despite his efforts to close down opponents, he was spectacularly unsuccessful (as if you can silence a Russian!) At his best, he saw the dialectical need for such struggle and thereby encouraged different views.