I agree, we should do this with Muslims too, right? Since they also kill people.
There is no debate amongst the vast majority of our society that neo-Nazis are provocateurs of hate, racism, and violence. Their beliefs (the suppression and murder of Jews, people of African descent, etc) cannot be tolerated in a civilised society.
The same debate about the role of Islam in terrorism in the Middle East and here in the US is nowhere near as settled. Therefore, I am not sure the slippery slope can be claimed here.