Possible Clues:

• Tripped over the criminal, by the sound of it
• That sounds as if one stumbled on a criminal
• Arsonist, for one
• Burglar, e.g.
• Someone with a record
• Big-time perp
• Sounds as if one has fallen upon evil doings
• One guilty of a grave crime
• One came down heavily, by the sound of it, on the criminal
• One deserved to be 18 down if one stumbled over this, by the sound of it