I had a similar character for an RP ages ago. (Radiation poisoning and needed blood transfusions in order to live) This character would basically knock out the person and steal their blood like a vamp, but through the use of an IV needle. This left the option for if I wanted to kill the character in question or just let them live whereas your trait seems to rely on death.

Another option is that over time you can add disadvantages like blood poisoning or many other medical conditions that may weaken your character. Let's say that they take the liver of someone dying of cancer. That liver may replenish mana but it could also create a very sick individual. It's similar to what [@oakman] mentioned about the sight of blood, but you can play that as Joker like card to make your character physically weak.

And yeah, ditch the shock collar. It just raises questions like, "Why has he got a shock collar?", "Why not get it removed?", "What happens when the batteries run dry?" or "Is a little pain worse than dying?". You've got an awesome skill that on it's own you can play with quite well as it's able to mix health, power, and mana into one bundle that has to be managed.

Generally the weaker you can make your character the more you can develop their personality and nature.