But I thought Google's patent play is a defensive play. So they should be looking for shields and armors not guns and bullets.

Mutually Assured Destruction is a defensive play... it's a recognized form of deterrence:

Mutually Assured Destruction is based on the theory of deterrence according to which the deployment, and implicit menace of use, of strong weapons is essential to threaten the enemy in order to prevent the use by said-enemy of the same weapons against oneself. The strategy is effectively a form of Nash equilibrium in which neither side, once armed, has any incentive to disarm thereafter.

http://en.wikipedia.org/wiki/Mutual_assured_destruction