Stygian drake managed to kill captured pirate

Gwen

Usually after capturing pirate summons and pets continued attacking pirate but did no harm to it. 
Today my stygian drake somehow  managed to kill it. 
Found it when returned to Tokuno captain to get reward.  Came back to ship and found this. 

On the picture below dead dread (lol) pirate is on my ship. 

Cinderella

I've had that happen to me
because my beetle poisoned it before it became invulnerable.
The poison doesn't go off of it, when the pirate gets binded