Let me preface this question by first stating that it is very obvious that WWII and the Holocaust were very very terribly bad. The worst possible. EDIT: It is also painfully obvious that Hitler was very terrible, but the question is about how much of the bad stuff was he personally responsible for, and how much of everything did he know?
However, just like how it is silly, but easy, to pin all the wrongdoings of a company on it's CEO, exactly how involved was Hitler with the concentration camps and war crimes? Can Hitler be blamed for every single war crime commited by the Nazi Regime. Clearly, he was not a good person, but how bad was Hitler?
TL;DR: Hitler was very bad, personally initiated many of the Nazi regime's crimes, and condoned, either explicitly or by inaction, those crimes he did not personally initiate.
Long version:
There are two concepts that we should cover to shed some light on this question.
They are "intentionalism and functionalism". This is a controversy in the historiography of the Holocaust, and concerns the extent to which the Nazi higher-ups - ie, Hitler and his staff - initiated the Holocaust. The intentionalist school is that Hitler, at some point prior to 1941, made a conscious decision that the Jewish people should be exterminated. A "hard" - though generally dismissed - intentionalist position would be that of Lucy Dawidowicz, who argued that Hitler decided as early as 1919 that he would gas the Jews in revenge for his own gassing in WWI! The functionalist position is that the initiative for the Holocaust came not from Hitler, but from the lower-ranks of the Nazi bureaucracy, who dealt with the "problem" of having to feed and control occupied Jewish populations by simply killing them, knowing both that nobody would care about the Jews and that Hitler liked the most "radical" solutions anyway, and that this received retroactive official sanction, and thus snowballed into an organized murder program. A "hard" - and also generally disagreed-with - functionalist interpretation would be that the Nazi leadership had nothing to do with launching the Holocaust, and it was an entirely "bottom-up" operation.
Neither position disputes that the ultimate moral responsibility for the Holocaust lies with Adolf Hitler and the Nazi leadership. Even if Hitler did not directly "start" the Holocaust, as the functionalists say, he created the climate of antisemitism, racism, and war that made it happen. After finding out, as he must have, he did nothing to stop it. Until the end of the war, the Nazi leadership made special efforts to kill Jews, sometimes to the detriment of fighting the Soviets (indeed, the Nazis would not have recognized a distinction between those two objectives).
There is now a popular synthesis of both positions on the controversy, which holds that Nazi underlings based their policy not on explicit orders from Hitler, but on what they believed he would have wanted. Hitler tended to give general instructions. Even if it was not originally Hitler's intention to exterminate European Jewry, once that process had begun he did nothing to disabuse his subordinates of their notion that they were carrying out the will of the Fuehrer.
The bottom line is this: Hitler may or may not have personally initiated the 3rd Reich's genocidal policies in relation to the Jews, Slavs, Roma & Sinti, and other minorities, but even if he did not, he created and encouraged the climate in which they developed.
More generally, Hitler can be blamed for Nazi warcrimes because he either started them or encouraged them by his policies and refused to effectively police them. A parable: If two GIs loot a French farmhouse and kill the farmer, it isn't Eisenhower's fault because the US Army had a robust and enforced system of military policing. If two SS-men loot a Russian farmhouse and kill the farmer, it is Hitler's fault because he issued propaganda and orders saying that Russians were sub-human, effectively abolished the concept of war crimes in relation to Russians, and put in place no systems to check criminal behavior by his men.
A book I think it would be helpful for you to read is Professor Richard J. Evans's Telling Lies About Hitler: The Holocaust, History, and the David Irving Trial. In 1996, lying holocaust denier David Irving, author of Hitler's War and a lying holocaust denier, sued Deborah Lipstadt for libel after she scandalously alleged in print that he, lying holocaust denier David Irving, was in fact a lying holocaust denier and had deliberately tried to exonerate Hitler from responsibility for Germany's WWII crimes. To conduct her defense - which essentially consisted of proving lying Holocaust denier David Irving was indeed a lying Holocaust denier, and so she could not libel him when her allegation was true - her defense team hired Evans to go through Irving's work with a fine tooth-comb and check his references and claims against the historical record. What Evans's dossier and testimony showed beyond doubt was that Irving had systematically lied and distorted the sources to "prove" his position.
The book is essentially a teardown of the premise that Hitler was not responsible for the Nazi regime's crimes in WWII, and so will be helpful in illuminating for you the extent of his responsibility.
Also: David Irving is a lying Holocaust denier. Should have made that clear.
Not to discourage further discussion, but this topic has already been discussed here.