Several points.

One. Even if your character doesn't tell anyone about his release, we need to know what it does for the purposes of balance.

Two. ...665 people? Really? Even in the 1400s, that is a lot of people, almost impossibly so. Seeing as normal people lived to their 30s on average, that is 20 people a year. Mass Murder as defined today is 4 people or more. I'd give a leeway of about 10-20 back then, but that is still way past my suspension of disbelief. Any smart ruler would send an army to kill this guy at 50-100.

Three... even if this guy killed 600 or 10 people, if he was that merciless, he really should be in hell.