Clear case of "motive justifies the means". I think in a free democracy, no one should block any propaganda, as it the responsibility of the individual to asses what to read and what not. In a democracy, it is more dangerous to censor and justify the means with motive - this opens the door to unjust censorship.
The best counter-argument I can provide to your wonderful ideal is that people are stupid, and they are vulnerable to being manipulated into believing dangerous peace-disrupting falsehoods by propaganda.
If the people are too stupid to discern propaganda from truth, then they are too stupid to vote.