A related concept is the association fallacy [1], which is a more subtle form of the ad hominem fallacy.
It works like this:
A. You believe X.
B. A crazy person also believes/believed X.
C. Since you believe X and a crazy person also believed X, you are a crazy person.
D. Optionally: I won't consider your argument unless you tell me why being a crazy person is ok. Why do you support doing crazy people things like being a serial killer?
Example:
A. You are against cigarette smoking.
B. The Nazis were also against cigarette smoking[2].
C. Therefore you are a Nazi.
D. Optionally: I won't consider your argument unless you tell me why being a Nazi is ok. Why do you support anti-semitism?
The latest updated version of this is: What you believe is arbitrarily labeled a conspiracy theory. People that believe in conspiracy theories believe the earth is flat. Why do you believe the earth is flat?
[1] https://en.wikipedia.org/wiki/Association_fallacy#Guilt_by_a...
[2] https://www.goodreads.com/book/show/251213.The_Nazi_War_on_C...
It works like this:
A. You believe X.
B. A crazy person also believes/believed X.
C. Since you believe X and a crazy person also believed X, you are a crazy person.
D. Optionally: I won't consider your argument unless you tell me why being a crazy person is ok. Why do you support doing crazy people things like being a serial killer?
Example:
A. You are against cigarette smoking.
B. The Nazis were also against cigarette smoking[2].
C. Therefore you are a Nazi.
D. Optionally: I won't consider your argument unless you tell me why being a Nazi is ok. Why do you support anti-semitism?
The latest updated version of this is: What you believe is arbitrarily labeled a conspiracy theory. People that believe in conspiracy theories believe the earth is flat. Why do you believe the earth is flat?
[1] https://en.wikipedia.org/wiki/Association_fallacy#Guilt_by_a...
[2] https://www.goodreads.com/book/show/251213.The_Nazi_War_on_C...