Diagnosis
The trigger switch micro-contacts are stuck closed or the control board's MOSFET for the light circuit has failed.
How to Fix It
First, try tapping the trigger sharply multiple times. Sometimes this frees a stuck contact. If that fails, remove the battery for 30 minutes. This resets the electronic control board. Reconnect the battery. If the light is still on, the switch assembly is likely faulty.
If That Didn't Work
It is a board-level failure. The tool will still operate, but the board needs replacement at a service center.
Heads Up
This will drain the battery if left attached. Always remove the battery when storing the tool if this is happening.