It's apish. Also the way his wife and daughter embrace, and when he joins them how they are posed, is designed to be anamilistic.
One of the great problems with the film imo is that it has muddied themes. Cady talks about reducing him to an animal level, and he mentions making them 'the same' (i.e., savage, having nothing left to lose) but this conflicts with the religious points it seems to be exploring as well as the ideas about testing the family's strength, and the lawyer's conviction in the law. It's all blended together so that there's no clear message imo.
However, that pose is very clearly part of their final confrontation's animal nature.