Just saw this tweet coming from a post on reddit.
Apparently if you hold down the trigger button while shocking someone it will kill them.
Haven't tested yet...
UPDATE
I tested this and it did work, but not the way I expected. When holding down the button, the defib attack seems quite fast, but it still took 3 hits in order for me to kill someone. I was aiming for the chest, but the player was prone, so I'm not sure if any of the attacks missed.