It would be critical to first define the term "Holocaust." We need to establish some common ground as to the definition of "Holocaust" before being able to respond. "The Holocaust" is quite often defined as the systematic murder of six million Jews by the Nazis largely through use of gas chambers. I would personally take a much more general definition of Holocaust which leaves out the specifics. Such an approach would be more similar to any other term to define a historical period. For example no one defines a war based on the number killed, the method of murder, or even the participants. I would accept the definition of "Holocaust" as the tragedy which befell European Jews during the Second World War.

I interpret your question to mean, "Was the systematic murder of six million Jews largely in gas chambers due to Nazi Ideology or improvisation in the camps." There was not a systematic murder of six million Jews in gas chambers, therefore the question becomes meaningless.

Similarly if we speak of the tragedy that befell European Jews, the question is also ambiguous at best. If we consider all elements of forced deportation, forced labor, disease, and mistreatment in the camps and out of the camps then surely some portion of the blame rests with Nazi Ideology, some with improvisation of field and camp personnel and some with the general conditions which persisted in Germany as a result of the closing days of the war.