Malbim: If the Mizbe'ach is for idolatry, do not ask why we wage Hashem's battle. Idolatry harms the entire nation, like we find regarding Pe'or. There was damage for generations - we still are not purified from it (54 years later), and damage at the time, i.e. the plague (also innocent people died in it)! Also your sin will harm us now and later (verse 18)!