A character would definitely be able to kill an NPC. What kind of IC consequences they face would depend on how you want to play it and the context. In previous events like the chalice ones, characters who killed others while under the effects were granted leniency, so if it is under the effect of the amnesia event, similar concessions could be made. There's also the option of the character not being caught, or being caught but having to face some kind of consequences (house arrest, or similar, something we could work out if you were interested in it).
In other words, there's a lot of ways to work around it depending what you're interested in!
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A character would definitely be able to kill an NPC. What kind of IC consequences they face would depend on how you want to play it and the context. In previous events like the chalice ones, characters who killed others while under the effects were granted leniency, so if it is under the effect of the amnesia event, similar concessions could be made. There's also the option of the character not being caught, or being caught but having to face some kind of consequences (house arrest, or similar, something we could work out if you were interested in it).
In other words, there's a lot of ways to work around it depending what you're interested in!