I'm thinking of suffocating my patient and letting him die.
Why would you let your patient die?
Because he is causing me harm.
You have a professional duty to save lives and allowing your patient to expire knowingly is a breach of that duty. Bring your patient back from the brink and save them.
I am not obligated to save him and I can do whatever I want with my patient.
You are obliged to save him if you took the Hippocratic oath.