Neither the precise scale of the killings or the degree of militancy of the protesters has been well documented. It's reported that a lot of policemen and revolutionary guard soldiers were killed too.

Remember, Mossad publicly boasted that they were on the ground with the protesters, which was a pretty insane thing to do and basically gave Iran carte blanche to say these aren't protests, it's a foreign sponsored coup attempt. There's very little we can say to that when Mossad basically publicly said it was.

Maybe they were so sure the protests would succeed they figured it would earn them/justify goodwill with the new government?