Orwell's book was entirely about state power; it was a parody of Stalinism. He was in the awkward position of being a socialist who'd been shot at by hardline communists during the Spanish civil war, and wanted to warn the left that Stalinism was really dangerous and should not be associated with.
Orwell was fighting for the POUM (Workers' Party of Marxist Unification) not against them, this faction had the support of Stalin. He was shot by members of Francos army, they were a Fascist / Nationalist party that had the support of the NAZIs and Fascist Italy.
I'd conflated two events; you're correct that Orwell was shot by an Fascist sniper, but there were also Stalinist purges going on against POUM ( http://en.wikipedia.org/wiki/Homage_to_Catalonia ) as it was a non-Stalinist Marxist faction.
One correction: POUM was anti-Stalinist. By the end of Homage to Catalonia, Orwell describes how POUM was driven underground by pro-Stalinist parties PSUC and PCE (I think these were the two).
POUM was definitely left-anarchist/communist though.
