Although I think the Fentanyl argument is just an excuse to gloss over his inherent craziness, I do think Duterte needs to be impeached. Killing people is not okay in any capacity, let alone as a President. His campaign against drugs is just crazy, how is shooting people going to solve a drug crisis? Some addicts need rehabilitation, they shouldn't be shot. The world is turning insane I'm afraid.
Any leader of a country has a job and that is to advance the progression of their country and keep their people alive and thriving. If a leader isn't doi this job then he either needs to change something of his processes or be impeached from office if he has bad intentions.
If the President has a drug problem, and cannot properly rule, then he should absolutely be taken out of office. Nobody can abuse drugs, prescription or not, and still be considered competent to stand up and govern their country. He needs to be impeached, or at the very least forced to resign.
Those who want to take out Duterte will say whatever they think will work to get him from power. Each President claims to want to impeach the one before him. Each President faces political attacks that become personal. Duterte should not be impeached because he is still doing his job for the people.