So the question is: if one believes in true socialism/communism without agreeing with dictatorships and anti-democratic states, how would that person be called?
Let's break this down through the multiple questions you ask.
This person would be called a "sectarian" for believing they have special knowledge of a future and as yet unrealised society, and holding their views above all others and above experimental determination of what is possible.
if one believes in communism
One is called a Communist or an Anarchist. There's a minor tendency going around to call for "Full Communism Now," as in "higher" communism, as in a stateless society immediately. Obviously a large number of Anarchists have been calling for this for over a hundred years.
if one believes in communism without agreeing with... states
Still a communist or an anarchist.
without agreeing with dictatorships and anti-democratic states
So there's obviously an area where some socialists or communists agree with some kinds of states. Let's break this down:
Support revolutionary councils of workers directly controlling their own society, and repressing the bourgeoisie
Council Communists, Left-wing Communists, some Bolsheviks / Leninists, Bordigists, some Anarchists (often dispute that this is a state due to a different definition of "state" to most Marxists).
The general term for these is either Anarchists, or, Libertarian Communists, or, simply, Communists.
Support revolutionary councils of workers OR state-capitalism governed by an ideologically pure party controlling society
This is generally Bolshevism / Leninism and its off-spring.
Support parliamentary style representative democracy either with direct workers council control of the economy or state-capitalist nationalisation in parliamentary hands
This is generally known as "Social Democracy", though social democracy has abandoned this programme, and a number of former communist parties have taken it up ("Eurocommunism" / "New Times"). This is also occasionally seen as a more maximal demand by Labour Parties in the British / Australian / New Zealand mould, though these have abandoned any element of this programme too.
Though, given the argument that bourgeois parliaments are dictatorships, either Bolsheviks / Leninists who advocate traditional forms of states (bureaucracies, parliaments, etc.) and continued wage labour (state capitalism) should be included here. Similarly the argument has been put that Social Democracy is anti-democratic and dictatorial because it ends up implementing the wishes of the bourgeoisie against the workers when it achieves power in a bourgeois parliamentary state.