The fascists will kill you for voting for their opposition sure. They were going to kill their political and ideological opponents regardless. They know they need to remove socialists and progressives so the country they takeover has no alternative to them. They aren’t going to spare a member of an out group because that person voted for them.

Nationalism is just a mechanism to justify racism to the fascist. Not all racists are fascists, but all fascists are racists.