If you say:

I do not agree with Hitler

You are saying generally, that in terms of your politics and opinions you and Hitler do not see eye to eye

If you say:

I am not agreeing with Hitler when describing his opinion

You are indicating, specifically, that in the process of describing his opinions about the Jews, you are not agreeing with said views, only describing them, and since the author presumably follows by describing those opinions, it is the appropriate time to provide that qualificaiton.

I do not agree with Hitler when describing his opinion

When to use "am not" and "do not" in a sentence

Has a good explanation of the difference

If you say:

I do not agree with Hitler

You are saying generally, that in terms of your politics and opinions you and Hitler do not see eye to eye

If you say:

I am not agreeing with Hitler when describing his opinion

You are indicating, specifically, that in the process of describing his opinions about the Jews, you are not agreeing with said views, only describing them, and since the author presumably follows by describing those opinions, it is the appropriate time to provide that qualificaiton.

I do not agree with Hitler when describing his opinion

When to use "am not" and "do not" in a sentence

Has a good explanation of the difference

If you say:

I do not agree with hitler

You are saying generally, that in terms of your politics and opinions you and hitler do not see eye to eye

If you say:

I am not agreeing with Hitler when describing his opinion

You are indicating, specifically, that in the process of describing his opinions about the Jews, you are not agreeing with said views, only describing them, and since the author presumably follows by describing those opinions, it is the appropriate time to provide that qualificaiton.

I do not agree with Hitler when describing his opinion

When to use "am not" and "do not" in a sentence

Has a good explanation of the difference

If you say:

I do not agree with Hitler

You are saying generally, that in terms of your politics and opinions you and Hitler do not see eye to eye

If you say:

I am not agreeing with Hitler when describing his opinion

You are indicating, specifically, that in the process of describing his opinions about the Jews, you are not agreeing with said views, only describing them, and since the author presumably follows by describing those opinions, it is the appropriate time to provide that qualificaiton.

I do not agree with Hitler when describing his opinion

When to use "am not" and "do not" in a sentence

Has a good explanation of the difference