They excommunicate members for a reason, which is why the organization as a whole can't be held accountable for the actions of someone who was hiding what terrible thing they are doing

In any organization or situation, the individual is always ultimately responsible for their own actions. This is true. However, you're making one big error. You're assuming that everyone disfellowshipped has done something wrong that would make sense in a greater (beyond the JWs) morality. You're also assuming, by extension, that the JWs disfellowship everyone who has done something morally wrong or illegal. These are both easily disproven by former member testimony. The JWs are fanatical about protecting themselves, and reporting criminal activity (like the abuse of children) is routinely forbidden.

What the real issue is here is how the organization itself chooses to deal with members who have done something wrong. That's the measure of a group, and in that, the JWs fail badly.