The argument/proposition usually goes something like this:

Hitler was a christian.... or Spanish Inquisition....  Stalin, Mao, Pol Pot.. Genocide....Athiests kill people and so on.

Just searching and reading i've seen the 'Atheist Atrocities Fallacy' which makes sense.  Even if all atheist were evil people and killed everyone that would not have any bearing on whether or not there was god or not.  But since I see both theists and atheists use it maybe there could be a better term? Or does this fall under a fallacy already?

Maybe just 'atrocities fallacy'?

I feel like i've seen it around more lately and it always is pointless and diverts from actual conversation.

Poisoning the well???

Edit:  You have given examples that could be used to commit the Poisoning the Well fallacy, but the context these examples are used in would have to be known, also.

Depending how it is used, it can be an Association fallacy.

For example, if one points to Communist atrocities as proof that atheists are evil.

A key way to recognize this sort of fallacy is figuring out if the atrocities are actually relevant to the point that author is trying to make.

Appeal to emotion.
Bad things were done by people who are/were atheists. Therefore atheïsm is bad. You cant possibly condone such acts, can you? You are a good person, arent you? You understand how dangerous this ideology is, don't you?

Among those above, it is also an Appeal to the Man (Argumentum Ad Hominem) Seeking to prove a conclusion false by attacking the character, reputation, associations, or social situations of the person proposing it, and Converse Accident: Generalizing from an exceptional case to proposed general principle, a common fallacy by Christians that hate atheism.  Solitary