If god's free will is limited by his nature, then he has no free will. And if you say he has free will but always wills to do good, then we too could have been created the same way and there'd be no evil and we'd have free will.
You simply cannot argue that human moral evil exists because god gave us free will and we therefore can chose evil actions. If god has free will and can only chose good actions, then there's no logical reason why we couldn't as well. You'd be admitting that "free will" is compatible with "always choosing to do good" and would be undermining the free will defense.