Itachi did many things in his life. Death was a good release of those regrets. He didn't have to worry about the pain and loss he felt after going against the Leaf Village to help Danzou destroy his clan.
He also tried to fuel as much hate in Sasuke as he could,so Sasuke would push himself to be strong. Itachi let himself be defeated by his brother,because he wanted to witness the strength he had achieved. He was also happy because his little brother,the shadow of the clan,was able to defeat and surpass the prodigy of the family.
He died happy,because of the release of painful memories and the pride and joy he felt for his brother. He died with a smile on his face.
