You should probably read the history of anarchism in Imperial Russia.

There were tons of assassination attempts against the tsars in the 1800s, and they even killed a tsar. Yet the empire fell due to an external war, roughly 100 years later.

Aristocracy became useless around 1600-1700, when gunpowder meant that peasant armies took over instead of knights, yet it took between 200 and 400 years for full legal equality to be introduced everywhere and some places still have nobility with special status, to this day, even in developed countries.

This is not a superhero movie. Fascism is a symptom, killing one individual will not do away with it. Also its relatively easy to divide revolutionary groups because they are very paranoid, whereas fascist leaders are often highly insulated by the state apparatus. But fascism is also not a very good form of governance and tends to collapse on its own after only a few decades at most. Its really more like, every so often, a bunch of people are needlessly executed to preserve the capitalist system, and then after that things kinda go back to "normal" for a while. Revolutionary movements operate somewhat irrespective of fascist regimes, although yes its more difficult to do so under an oppressive state.