I was wondering that if you pickpocket the briar heart (if possible) does it result in instakill?, it should i think.

i think it does. I believe I did this to one on one of my many times playing Skyrim. Snuck up on him and just took the heart and he died...

Thanks, i really need to try it sometime

Wait...what?!

Yes it does.

There are certain forsworn mages that are called Briarhearts. They have their chests cut open and crudely sewn with a Briar Heart item instead of an actual one. You can steal it from them and they die instantly.

There is a bug though. Sometimes dead briarhearts send hired thugs after you for stealing from them.