Celebrim
Legend
I would think that the commoner had no standing to arrest, imprison, or assault anyone - noble or otherwise - regardless of the situation. Commoner has asserted magesterial authority and placed himself above the law. This is unlikely to end well.
Commoner faces whatever punishments are meted out for false imprisonment and assault. At least some of these charges probably don't depend on the noble's guilt! Noble probably asserts that the allegations are false. Court probably finds that lacking any evidence from a credible source - the commoner appearing to be little better than a madman and is certainly a criminal - that no charges are to be brought against noble. Further, noble sues commoner for slandering his character, and lacking evidence of his guilt other than heresay, probably wins.
Commoner faces whatever punishments are meted out for false imprisonment and assault. At least some of these charges probably don't depend on the noble's guilt! Noble probably asserts that the allegations are false. Court probably finds that lacking any evidence from a credible source - the commoner appearing to be little better than a madman and is certainly a criminal - that no charges are to be brought against noble. Further, noble sues commoner for slandering his character, and lacking evidence of his guilt other than heresay, probably wins.