Uh, yes, you can. People do it all the time.
If you insist to argue, then I'll argue that the sword will be a constant reminder of what happened. Purging the memory from his mind would be pretty impossible. If you really want him to go mad, don't worry, lack of human contact is enough to do that. Besides, if he convinces himself that he never killed an innocent imperial, he's less likely to go mad.
Just let it flow. Flow like the wind, man. :celebration: