If murdering Nobunaga was Phase One of a long thought-out and well-considered plan for the take-over of Japan it certainly wasn’t obvious in 1582.  In fact, it hardly seems obvious today!  One thing we do know, Hideyoshi, though flat-out besieging the Mori clan, was not going to let this opportunity pass.  If things went right, he could achieve victory over the Mori, avenge his lord and overcome rivals for control of post-Nobunaga Japan.