I left in changed form, such as Mr Hyde was known to do (10)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

I + L (left) contained by [in] MUTATION (changed form). I can’t remember the gruesome details of Mr H’s deeds but a summary I found on line advises: “His actions are characterized by a lack of empathy and a willingness to inflict pain and suffering on others” so I assume this may have included mutilation.