It's not self-defense if the fascists haven't done anything violent or destructive. Unfortunately the fascists in the US have been doing violent and destructive things and are close to completing their takeover of our democracy. As an exmaple, there were January 6th rioters and organizers who were arrested and imprisoned. However, there are plenty of people who would meet criteria that would make them varying degrees of fascists, but haven't done anything yet.

There are overt fascists who espouse fascist ideology and talking points, but haven't performed acts of domestic terrorism like January 6th. There are people who want their prejudices validated and want to return to an imaged past. There are people who have bought into the lie that minorities are the cause of their problems instead of systemic issues. There are people in the Republican Party who are neo-cons who want to work with the fascists. There are people who shout both sides further to the left of the neo-cons. There are neo-liberals who want to keep things the same despite our downward spiral into fascism.

A christo-fascist regime is being built primarily thanks to the actions and inactions of people using our system without directly hurting anyone or destroying any infrastructure. The fascist movement is spreading ideas, violating democratic norms, upending the guard rails in our democracy, filling the government with like minded individuals, and in the case of Trump, violating the law and stalling out the court system. The Supreme Court ended the rule of law on July 1st, 2024, not by shooting people, but with the stoke of a pen.

The fascists build the system to target groups for elimination in a way that doesn't directly harm anyone until the system is in place. Your argument calls for the elimination of people based on political ideology.

they tend to target the least dangerous people first.

The enemies of fascists are at the same time both too strong and too weak. They claim the out-groups they target are both inferior to and a threat to the in-group. According to the fascists the out-groups must be eliminated to preserve the in-group. And the in-group is justified in eliminating the out-groups because of the in-group's superiority. This is a contradiction, because a superior group should have nothing to fear from an inferior group.

The fascists divide the population into in-groups and out-groups as a mechanism of control, usually based on physical characteristics. As part of their crusade against out-groups they actually have to destroy the out-groups. But the fascists need an out-group to exist in order to stay in power. If there is no threat to the in-group, then there is no use in having a fascist strongman.

To solve this problem, fascists have to repeat the process of dividing the population into in-groups and out-groups. The Nazis divided people into a extensive racial hierarchy from the get-go. This process repeats itself until no one is left. It is not a slippery slope, but rather the nature of fascism as a self-destructive ideology. Inevitably the fascists are defeated military or destroy the entire population. In the end, no one meets the ideal qualifications of a superior human that the fascists imagine.

Your argument does the same thing with ideology. It calls for the preservation of an in-group by the extermination of an out-group. The out-group is both a threat to the in-group and yet inferior. Rather than striving for purity of the body your argument seeks purity of thought. It is the same outcome sold with a different scam. Once everyone considered to be too fascist to be allowed to live has been killed the next out-group will be on the chopping block.

There's a popular adage that says, scratch a liberal and a fascist bleeds. The neo-liberals will be deemed to dangerous to keep alive because they obstruct systemic change and their ideology leads to fascism. The progressives will be deemed well meaning, but under closer examination they aren't left enough. Their policies don't allow workers to seize the means of production.

The social democrats, while socialists, believe in democracy. And what is democracy to the far left but an incubation chamber for fascism. The democratic socialists are also open to a political socialist revolution, ie democracy, and cannot be trusted by the far left. What are socialists but people who believe in an ideology that is only a stepping stone to communism. Communists who want a stateless society are simply deluding themselves according to the authoritarian communists. The authoritarian communists believe only an authoritarian dictatorship can preserve the worker's revolution.

We move so far left we end up wrapping around back to the far-right in the form of authoritarian communism. The most substantial difference between the red fascism your argument is proposing and the fascism we are seeing in the US today is that they divide people on different metrics. These ideologies are all self-destructive because there is no stopping point. No one is good enough by any metric, and so what these ideologies are really saying is that we are all better off dead. No one deserves fascism because life is worth living no matter our flaws.