Possibilities:
1. Better the devil you know...
2. His second in command might be more competent/more evil/better at the job/better connected/etc.
3. Might have been harbored by someone, might have really good tradecraft or even just good luck.
The above are standard reasons - no idea if they actually apply here as I am just speculating.
Well, I figure if they can kill the number one guy, they can find the number 2 through 20, and kill them all. The world would never miss them. 👍