Is a Metzora a Sinner?

Is a Metzora a sinner?

The obvious answer is yes - he is a sinner. The Mishna in Shekalim 5:3 aludes to the fact that he's a sinner. Not to mention the Gemara in Arachin 16a that tells us the 7 sins that are punishable by Tzara'at.

If so, we have a difficulty. The Mishna in Menahot 9:6 say that the Korban Hatat and Asham of a Metzora both require Nesachim (libations), even though every other Hatat and Asham do not. The Bartenura there explains the reason for this: "because they do not come for a sin like other Korbanot Hatat and Asham. The Hatat of a Nazir does not require Nesachim because a Nazir is a sinner - as it says מאשר חטא על הנפש - because he caused himself pain from [not drinking] wine."

This would imply that a Metzora is not a sinner!

However, the Tosefot Yom Tov there resolves the difficulty by saying that what the Bartenura means is that a Metzora is definitely a sinner - just his Korbanot Hatat and Asham are not brought because of a sin.

My question is that if they are not brought because of a sin, why are they called Hatat and Asham?

(Note that the Ramban on Vayikra 14:18 seems to suggest sins for what each of the Korbanot of the Metzora atone. I wonder if we're forced to say that he argues on the Bartenura's explanation of the Mishna.)