What did Karl Donitz do and who was he? Why did he do it?

Karl Donitz (alternate spelling: Doenitz) was an influential leader of the German Navy during World War II and assumed the leadership of Nazi Germany during the final weeks of the war in Europe following Adolf Hitler’s suicide.
The extent to which Admiral Donitz was a committed Nazi with respect to Hitler’s racial theories as opposed to being an extreme nationalist is uncertain given his history as a serving officer in Germany’s navy. What is know for a fact is that he was instrumental in Germany’s circumvention of the limits on Germany’s post-World War I remilitarization following the conclusion of the Treaty of Versailles. A submariner by training, he was committed to the surreptitious rebuilding of Germany’s submarine fleet and was responsible for that fleet’s design and use with respect to attacks on shipping across the Atlantic Ocean. He was the innovator of the use of German submarines, called U-boats, in concentrated packs (labeled “wolf packs”) that successfully sank an estimated 2,779 allied ships representing over 14 million tons of shipping. In addition, Donitz was known for the savagery with which he executed his mission of disrupting Allied supply lines and sinking enemy vessels. He was absolutely ruthless in demanding the killing of Allied servicemen who otherwise survived the sinking of their vessels.
So loyal to Hitler was Donitz that the German leader designated the admiral as his successor following Hitler’s planned suicide. Because of his committed service to the Nazi regime and the ruthlessness with which he carried out attacks on Allied shipping and servicemen, Donitz was tried as a war criminal and sentenced to ten years in prison, following which he lived out his life as any retired naval commander, including openly associating with other retired naval officers from both sides of the war.
The “why” as to Donitz’s commitment to the German war effort was relatively simple. He was educated and trained as a naval officer specializing in submarines and submarine warfare and carried out his orders to the best of his ability. Unfortunately for the millions whose lives he adversely effected, he was very good at his job. Like many in the German military establishment during and following World War I, he was determined to avenge the indignities he perceived as having been inflicted upon his country in the Treaty of Versailles.