It is not necessarily the case that Floyd died from a knee on his neck. His official autopsy (not the "independent" one paid for by his family) shows that he had a potentially lethal level of fentanyl in his system (11 ng/mL) in addition to 19 ng/mL of methamphetamine (https://webcache.googleusercontent.com/search?q=cache:bPUNU7...). That combination of drugs is one variant of what's known as a "speedball". It can cause death by respiratory failure. https://www.addictioncenter.com/drugs/heroin/speedball/. The autopsy also says "No life-threatening injuries identified", in particular "No injuries of anterior muscles of neck or laryngealstructures", so it is unlikely that the death was caused by Derek Chauvin putting his knee on Floyd's neck.

There is also the problem of motivation. What possible reason could Derek Chauvin have to murder George Floyd? Surely he understood that doing such a thing could cost him his job, risk imprisonment, and make it very hard for him to find employment. Much more likely is that he believed the hold he used was nonlethal, and, given what we know from the Hennepin County autopsy, that it actually was nonlethal.