Evil is a religious concept.
Selling drugs isn't evil. Not selling drug doesn't make you good. People take drugs for various reasons. If a doctor sells them they are good but if someone else sells them they are evil?
The person buying could have been fired and can't afford Doctors prescription so the person selling could be an angel.
Fentanyl takes the fun out of drugs and since its laced in every street drug these days it means that no drug is safe anymore.
It cannot be understated how harmful fentanyl is and how low quality of a drug it is. Low quality as in the high sucks.
(I've never taken drugs and I don't drink)
Evil is a moral concept, which is less tied to religion these days.
Drugs are an anti-social drain on society, that sickens its buyers, turning them into zombies or criminals, and turns the sellers into greedy, violent people who corrupt law enforcement.
Your edge case of an angel doesn't translate to the actual realities of drug trafficking and addiction.
A doctor that over-prescribes them would be arrested, too. Or one that prescribed it to someone for a non-medical reason. (There are many of those latter docs.)
People that sell fentanyl (or similar) are very bad for society, to avoid the triggering "evil". Look how many people have died in the last 10 years. It's insane.
EDIT: I personally know a young man that died from a fent overdose and it's likely he didn't know what he took had fent in it. 22 years old and the whole world ahead him. Completely destroyed his family.