What fallacy is this?

In some christian-atheist debates I heard the following argument made by some atheists:

In the name of religion/God many atrocities were committed
It is immoral to commit such acts in the name of religion/God
Therefore, God does not exist.

Note: This is a prototype argument using religion to refute the existence of God. I don't claim that it was not employed in this form by some debater.

It is begging the question and non-sequitur. The last sentence is non sequitur; just because some people committed atrocities - whatever that is - in the name of religion, it does not follow that there is no God. They may not represent the true God if there's only one, or of any god's point of view if there are multiple gods. It's also possible for the God or Gods do exist but are sadistic monsters who enjoy torturing people or having them tortured, the way some kids would pull the wings off flies.

It's also possible the God or gods may simply be indifferent to human behavior.

As for the second point, whether the actions of religious groups were atrocities depends on what you decide is horrible. Some might consider the death penalty a form of barbarism while others might consider it simple retribution. But the concept of respondeat superior - the master is responsible for their servant's actions - has not been established in this case, so the idea that a God or gods is responsible for their follower's actions has not been proven to be applicable. and it hasn't made the connection as to how the commission of atrocities by followers of a God or gods has any relevance to the existence or nonexistence of that entity.

The Coca Cola Company operated a subsidiary in Nazi Germany during World War II; the subsidiary had to use scrounged materials to keep operating while commerce was suspended. The fact a subsidiary operated in a totalitarian country does not disprove the existence of the Coca-Cola Company.