I am not really certain what you are referring to.

You may be under the impression that the favorable outlook towards fascism of various former socialists and revolutionary trade unionists betrays a convergence of positions between socialism and fascism - and if the "fascist program" of 1919 in fact contained a few themes derived from the "maximalist" wing of official socialism, those threads had long since grown apart.

To a first severe division in the socialist field in 1912 with the expulsion of the so called "reformer" - or "revisionist" - wing of Bissolati and Bonomi, a much more deep fracture had followed the interventionist debate that had moved a bunch of former socialist (yes Mussolini was the most famous of those), either out of nationalism or following the new myth of the revolutionary war, on the vehemently interventionist positions of the nationalist-trade unionist-futurist block.

Those men had thus separated from the official socialist group (of Lazzari and Menotti Serrati for the intransigent portion and Turati and Morgari for the Parliamentary group) - that remained neutralist at the time - and even more from the Marxist-(sort of Leninist) wing that would later found the Communist Party of Italy (in 1921, two years after fascism was created).

Those former socialist, trade unionists, etc. fell into the lines of the Fascio Parlamentare di Difesa Nazionale or within its outskirts after the Caporetto defeat; an organism more and more dominated by the radical right (in the Italian context of the time) and financed by the heavy industry and protectionist fraction (the notorious relations between Ansaldo and Mussolini for financing his Popolo d'Italia).

After the war, the "no longer socialist" groups, left hanging after the falied attempt of the patriotic "Socialist Union", scattered all over the political world in a plethora of unconsequential small and smaller formations. The first fasci were among those, surviving with sporadic funding and a few dozens of members, while Mussolini himself semmed more interested to a "Party of Veterans".

In 1919-20 though, the socialists met with a large electoral success, especially in local elections of the Po Valley area, where they already held a vast and - frequently not entirely legal - dominance over the land workers league.

There the reaction of the land owners begun, funding conspicuously the local fasci which saw an unprecedented rise in numbers and begun a systematic practice of anti-socialist violence; at first more or less spontaneous unorganized violence, targeting the leagues and the socialist papers. Then violence against the elected administrations with the goal of forcing their dissolution (a tactic already applied to the leagues that often fell apart because no one was willing to take leading positions under explicit threat of death), while public authority was either powerless or conniving.

The result was the functional end of the socialist party, predating of a few years its official end in 1926; a violent death repeatedly denounced by many Socialist leaders - the most famous, that Giaomo Matteotti murdered by the fascists in 1924.

As evidence of this there is also the remarkable numbers of socialsit emigration to France in the years 1921-26.

I will go into more details if you have specific point in mind. But I am somehow unsure what your original focus was, as your question seems to imply something that is simply not true. As anti-socialist violence and opposition to the "mounting red tide" was the core element of fascism as soon as it became a relevant political force.