the moral equality of combatants

I've been thinking about war and the conditions under which it is obligatory for a combatant to refuse to fight in a war. Traditional Just War Theory has as one of its principles the principle of just cause. This principle basically says it's only permissible to go to war provided one's cause is just.

Certain theorists (e.g. Michael Walzer, Dan Zupan) argue for a principle often called the Moral Equality of Combatants (hereafter, MEC). The principle says all combatants, whether fighting on the just side (if indeed there is one) or the unjust side, share the same moral status with regards to jus ad bellum--the resort to war. One counterintuitive result of adopting this principle is that a combatant cannot be held blameworthy for conducting acts that essentially amount to murder since warring in the name of an unjust cause doesn't give one a justification or excuse for taking human lives.

I have a crude argument against MEC and i'm wondering what people think of it. It goes as follows:

Argument against MEC

1. Combatants have a moral obligation to not go to war for an unjust cause.
2. If (1), then combatants who go to war for an unjust cause are morally blameworthy.
3. From (1) and (2) it follows that combatants who go to war for an unjust cause are morally blameworthy.
4. Combatants who go to war for a just cause are not morally blameworthy.
5. Therefore, MEC is false.

I'm aware that Dan Zupan argues that combatants on the unjust side cannot be held morally responsible for jus ad bellum because of invincible ignorance--i.e. combatants epistemic states are such that they lack the requisite knowledge to determine the justness of a war effort. Provided i can answer this objection (and i think i can), are there any other worries about the argument?