Agreed that murder is not in itself madness, but this event was not just murder.  It was murder of a group of innocent people praying.  The murderer was not threatened or seeking gain or, as far as we know, personally familiar with the victims.  We might speculate that he had been filled with hatred or resentment by reference to his web browsing and posting habits, but many of us have strong views without feeling the need to pick up a gun.  Madness, or something akin to it, seems very likely to have played a role in his decision making process.