I like the idea of Tom dropping the Riddle last name and taking the name last name of his mother....Tom Gaunt. A pureblood last name, he has the parseltongue ability and family ring to back up the claims, no one would think to question him.

Now, what happens to muggleoborns like Hermione. Obviously they can't enter this pureblooded society. Are they taken from their parents? Does Voldemort order a secret mass execution of detected muggles using magic? (sorta like the Pharaoh's killing the Hebrew babies)

I think that Harry is going to need more than just the massacre to fully sway him. He is going to have to dig into history for information about Tom, the government, the order etc. Maybe along the way he discovers Tom approving dark, evil research of the worst kind. Research on living humans, animals, etc, all in order to discover/create more magic.

I think in FMA Roy also investigated his own government before taking action.

 

You don´t need the religion thing. It would only complicate your story.

I really liked your original idea of Harry forming an terrorist group opposing Riddle.

The followers for this are not that difficult to find. Think about this:
*Riddle gains power giving the pureblood more power...He gains power making bullies. And this will start a escalade of violence.

*He will make mistakes, and if not his "minions" will. He would need more violence to cover for this.

*Some honest and innocent people will get hurt. Someone is going to complain. You have to silence them. Censure. And violence because someone is not going to accept it and the easiest way is by killing them. Families, friends will remember. More violence.

* You lost your freedom. Young people is capable of doing ANYTHING for it. ummm violence?

*Some ambitious people would like Riddle´s power. And they are used to violence by now...so there you got it, more violence.

*dictatorships are expensive. You know all that secret detentions centers and aurors you need to keep yourself in power. Were I´m going to get my money from?..uhmmmm...There are a lot of incompetents in my government(your faut Tom you put them there because they were pure and followed you not because they were any good at what they did..) My economy is not the best, people is getting angry...More violence.

*Riddle cant have people questioning him, so he brainwashed the young. This lower the level of education, not in an academic way but you don´t want them thinking,do you? so no moral education...more violence?

*By now he becomes very paranoic ..so...more violence.

*Power have gone to your head and you are starting to development megalomaniacs attitudes...your allies are getting afraid of you(more violence to "silence" them) this weaken your power base...you need even more violence now that you are few...And let´s not forget that megalomaniac attitudes tent to be expensive..

*I could think more reasons but I´m getting tired...

People live in a constant state of fear... they will accept anything if you promise them peace...even more violence....and well vengeance.



So there got your power base for Harry...with a real life/History flavor for you...

 

I doble post and I don´t know how to deleted....