Can I kill people with a defibrillator?

You can, but it's very hard. It requires at least two hits from full health, and only certain parts of the body will take damage (mainly the chest area).

Because of these two things, you'll only get kills on either really low health people, or people who are completely oblivious, and even then it'll require pretty good accuracy on your part.

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.