Fentanyl slows the respiratory system, and a lethal dosage was determined to be in his bloodstream. Did Chauvin’s unnecessary actions expedite his death? Absolutely, so yes, you could technically say he was murdered, but he wouldn’t have died had he been sober because the pressure applied to him wouldn’t have been enough to suffocate him had he been sober.
Actually you should list the pressure being put on your neck by that officer restricting your airflow and suffocating you as the cause of death. Fentanyl definitely worsened the effects of suffocation but you still list the suffocation as the cause of death there because it was direct physical impacts.