Funny thing, just watched Midway last night.

My understanding of him is that he knew Japan would not be able to win a drawn out war of attrition.  That's why he planned the Pearl Harbor operation and later conducted operations to try to draw the remaining US Pacific Fleet units into a decisive battle.  He hoped he could cause enough damage early that the US would accept ceasefire terms with Japan's holdings intact.  If that didn't work, they were to rely on defensive rings around Japan destroying as many Americans as possible.  He most likely also knew it wouldn't work.  He studied in the US and knew some of the American spirit.  Japan never had a chance but he was obligated by honor to do his best for his country.  

There's a lot more to it but that's the gist of it.

Read a book on the admiral years ago.  He didn't want war and his sentiment was known. Hence his removal & appointment to the fleet  where the Army couldn't assassinate him.

Japan's whole strategy in the Pacific was based upon their decisive victory over the Russian Navy at Tsushima in 1905.  The battle was a decisive and victory and right after it Russia came to terms.  They figured that that was how you won wars.  One decisive battle and when you win it the enemy will beg for peace.  The point of Midway was not so much to capture the island but to draw the American Navy into the decisive battle that Japan would, of course, win.  

Yamamoto knew that the Japanese Navy was not built for the long term.  They simply did not have the capacity to fight a long war.  The amount of fuel that that Japanese Navy used during the Aleutian and Midway campaign alone represented the total fuel budget for a year.   Their problems were twofold.  Their early successes covered a large portion of the planet, but most of the area was water with very little int he war of natural resources.  Far flung bases needed to be kept supplied and the Japanese never had a large merchant marine fleet.  Most of the goods going into Japan came in on foreign owned ships.  When they attacked Pearl Harbor they also did the equivalent of sinking millions of tons of their merchant marine fleet as most of the foreign owned ships were no longer available for them to use.  

Their second problem was that the Americans would not surrender.  We had been trounced at Pearl Harbor and in the Philippines.  Their plan book said we would beg for peace at this point.  On top of that the Doolittle Raid was a tremendous slap in the face to the honor of the Japanese Navy.  the life of the emperor had been threatened and they totally missed it.  They figured that they needed one more decisive battle.  Midway, they thought, was too important to the Americans for them to let it go.  We would scramble the fleet which would be disabled by air attacks form their carriers and then totally destroyed in a follow up surface engagement.  

There is a school of thought that since the itinerary for Yamamoto's flights had been transmitted in a code he knew had been compromised that in visiting the south Pacific he was deliberately choosing an honorable death in battle.  

As to Nimitz, he was probably the only Admiral the the US Navy that could have won the war for us in the Pacific.  He kept his cool under pressure, was aggressive where a chance at victory could be achieved and commanded the respect and loyalty of those under him.   On top of that he could also deal with MacArthur and was not intimidated by him.

He didn't have any problem with War against China for resources.

He knew that Japan could not win a long war against the material superiority via production against the United States.

The Goal was a "Shock & Awe" Blitzkrieg to take the needed territories (particular Malaya for Rubber and Dutch East Indies (Indonesia) for oil).  A hard one two to Knock out the Pacific Fleet at Pearl Harbor & Destroy the American Air Power & Naval Units in the Philippine and Western Pacific to take those territories for Japan.

If the Philippine Islands hadn't been smack in the middle of the shipping routes between those targets & a Japan, the Japanese would most likely have ignored America.

But because the Philippine Islands perfectly interdicted the critical supply route to the Homeland for Japan, the die was cast for war.  

Previous Wars, (like the Russo-Japanese War of 1905) , the Japanese had achieved several lightning defeats of a major "white" Western Superpower and thought that the Americans would fold with the loss of "unattached territories in the far Pacific", as long as their home land was not in danger.  

Yamamoto knew it would either be a quick victory or a long and bloody defeat.