It was the combination of the two drugs, him working himself up and atherosclerosis (clogged arteries). A stimulant like meth will increase your heart rate, along with the carrying on he was doing. The amount of fentanyl he was on would have severely depressed his respiratory system. A fast heartbeat+ slow respiration=death due to fluid building up in the lungs. Add a bunch of clogged arteries into the mix, you re toast.