That's running with the assumption, though, that the amnesia is permanent. If we're being anal about it, it shouldn't even be called amnesia, because that's not what it is. Magic doesn't exist IRL,* let along magical memory wipe, so there's no precedent for what to do with someone who commits crimes then literally has their memory destroyed. That's not 'just' amnesia. That is, for all intents and purposes, pressing the reset button on who they are as a person (or at least how they've developed as a person for X amount of time).
We also don't know if what happened to him is permanent or not. Those memories could just be forgotten, or they could have been erased and destroyed outright.
If they are just submerged or locked away, then yes, he should be held accountable to some degree. But if they've been flat-out destroyed or erased from his mind, then he is, for all intents and purposes, a total innocent.
* And if it does, I'm going to be PISSED. That totally should have been brought up in school vocation day. If Witch/Wizard is available as a profession, then you need to fucking tell people, so they don't waste their life doing things that don't involve dragons, magic swords, forever wars between angels and demons, and forging pacts with anthropomorphic personifications of nature.