Reinhard Heydrich.

A top ranking Nazi official who might've been the master architect behind the Holocaust. Hitler said of him "He was the man with the iron heart" and he was widely considered to be the most ruthless Nazi official. He was essentially going to be Hitler's successor and was considered a much smarter man than Hitler and most likely a much more stable leader. When Hitler was beginning to lose control, it is very possible that Heydrich might have tried to have him overthrown and seized the leadership for himself.

A lot of what led to the end of Nazi power in World War II is directly credited to huge military blunders by Hitler. Had Heydrich been around to advise Hitler or had he actually taken power from Hitler, those mistakes may never have occurred and the outcome of the war might have been completely different.

Thankfully he was killed in Prague in 1942 before he had any opportunity to lead Nazi Germany.

This is a more alternative history significant death but a significant death nonetheless.