Why did they mention Achan?
Malbim: Do not ask why we wage Hashem's battle. Your sin will harm the entire nation, like Achan's. Also he sinned against the Mikdash (the spoils were for the Mikdash), and there was anger against all of Yisrael (and Hashem removed his Hashgachah). All the more so this will happen if not only an individual sins, rather, two and a half Shevatim!
What is the meaning of "Ma'al Ma'al"?
Radak: The first is a verb; the second is a noun (he violated an infraction).
Why does it say "Lo Gava ba'Avono"?
Radak: He did not die alone. This is like (Melachim 2 6:10) "Lo Achas v'Lo Shetayim" (rather, many times).