As an ex-AXA Rosenberg employee (hence the anonymous posting) and having met Barr quite a few times through work, I probably have a better understanding than most of the situation.
The Research Centre was a self-contained unit (mostly operating just outside SF) which supplied data (basically trading instructions) to the AXA part, which made investments based upon that data. This kind-of exempts the AXA side from any blame, although ideally they should have had tighter reins on the RC.
Barr is not the kind of guy who you could question: he did what he wanted. Not in a authoritarian way, but purely by the fact that you never saw him and he had such an aura around him that you just didn't question him. He had access to the source code, as did many others, but notice that he specifically takes the fall for it ("Rosenberg directed the others to keep quiet").
Barr was not the only one that was punished internally for this - quite a few of the directors were moved on or retired.
Also as a result, AXA now owns the full 100% of the company which was something that it wanted for a long time after the original deal.
The model that the RC used is a behemoth written in Eiffel (no, I'm not kidding!) which was constantly undergoing revisions on a daily basis. The fact that there were errors would not be a surprise to anyone in the industry, but the scale is what is amazing.
Would Barr's bonus have covered the fine? Look him up on Wikipedia. It wouldn't have made a dent.