Of course individual and group behavior is quite complex (psychology, sociology). But my main take is that the group leaders set the tone for the group like bird flocks. Then the group being one of the main powers that makes up society, such as political, economic, military or religious will act in such a way to enhance its role. Usually these groups start by having a co-operative type behavior since they are weak and they need to be established. But as soon as they become strong their tactic changes and will lean toward more aggressive behavior so as to keep the status or enhance position. Such a technique is very humanly like. And the leaders are well versed in this technique as individuals(That is how they became leaders in first place), so they use that same game to enhance their groups.
Examples are abounded; just watch how rich people typically resort to arrogance while poor people are more humble like.
Even all religions started in a humble way, and as a matter of fact they all started as a reaction to some injustice in society, slowly negotiating and gaining power. It is only when they became powerful that that their aggressive behavior starts. And that is true for all other power pillars in society. And society as a whole also acts in this fashion and so do nations and group of nations.
Sure, from a psycho-sociological perspective, there is a point about mob mentality. People, when part of a group, will possibly do things (such as acts of extreme aggression) that they would never do on their own.
Over time, this mob mentality can get codified and a person adhering to the codex can manifest mob mentality even when not immediately part of a group.
But here, we are talking about theists. If God exists and is person-like, this means that He is watching all this shit going on in His name done by theists, and is just letting it happen. To boot, He apparently set up and runs the Universe so that non-theists are fully dependent on theists for all input on "God."
This is what I can't quite digest.