The case for reasonable doubt...

Keep in mind that in order for the prosecution to prove any of these charges they have to present enough evidence to prove two separate issues beyond a reasonable doubt:

• That the actions of Officer Chauvin was what provided the substantial contribution to the death of George Floyd
• That Officer Chauvin had a reasonable understanding that his actions could lead to the death of George Floyd. 

If a juror has a reasonable doubt that George Floyd died of positional asphyxia and instead believes that it was an arrest of the heart from underlying health conditions, the drugs in his system, or even a combination of both, then that should be enough for reasonable doubt. The fact that Chauvin may have contributed in some way is not enough under the law to say he committed murder or manslaughter. 

Likewise if a juror has a reasonable doubt that Officer Chauvin understood that the prone position restraint would likely cause enough physical harm to actually kill someone, then even if that juror believes that Floyd did die of asphyxia that would also be enough to provide reasonable doubt. 

There are a few key factors from a logical objective standpoint to consider when making a determination. For the first point. Cause of death. 

• George Floyd did not officially die of asphyxia. His death certificate states cardiopulmonary arrest, based on a combination of factors (none of which were proportioned within the official autopsy). The official medical examiner (the only objective doctor testifying) rejected the notion (under oath) that Floyd was asphyxiated. 
• George Floyd underlying artery blockage is considered coronary artery disease. This is generally labeled under the broader category of coronary heart disease. Coronary heart disease is listed on over 300,000 death certificates a year. 
• George Floyd had enough Fentanyl in his system to justify overdose as a cause of death. Doctor Baker, Doctor Thomas, and Doctor Fowler all testified under oath that if the examination of George Floyd focused just on what they found in the autopsy and toxicology reports, that they would have labeled this an overdose. Opioid overdoses account for the cause of death on 80,000 death certificates a year.
• There is almost no documented cases of positional asphyxia in healthy adults. It's been reported that about 100 people official died from a restraint asphyxia over the past decade and only 5 of those cases involved someone who was not obese, had not been tasered in a police stop, or did not have drugs in their system. Five people out of the thousands of suspects that have been placed in a prone position without issue over the past ten years or so. 

So would it be "reasonable" to believe that the official autopsy and official cause of death was correct and that the coronary artery disease, the 11 ng/ml of Fentanyl, or a combination of both was the underlying reason that Floyd died. Would it be "unreasonable" to suggest that causes responsible for 400,000 deaths per year might be a better explanation than restraint or positional asphyxia which has only killed five healthy people in ten years? 

For the second point. Intent.

• There was no real motive provided for why Chauvin would have had intent to harm Floyd. While there was much quibbling over excessive force, excessive police force cases are generally a civil, rather than a criminal issue. It becomes a murky grounds to argue that intent to commit third degree assault can just be assumed from contested excessive force.
• The criticism regarding turning to the side was made quite clear by many trainers and people who held that particular opinion. The prosecution failed, however, to prove that Chauvin had ever attended a class or had training where this technique was taught. The defense, however, provided a police trainer who suggested that while this was now part of new cadet training, that it was unlikely that veteran cops would have been exposed to it.

Bottom line is that you do not have to like Officer Chauvin or even think he is without fault here. But this is neither a popularity contest or an internal personnel disciplinary investigation to determine if Officer Chauvin followed his job description. The state could have handled this in any number of ways, but they choose to charge Chauvin with murder and manslaughter. They sought out to prove beyond a reasonable doubt that the official cause of death was incorrect and instead it was murder by assault or manslaughter with grave indifference. They have to prove "murder" or "manslaughter" to warrant a conviction. They cannot just prove that Chauvin is an unlikable and arrogant cop who might have missed some steps in the official training reference manual.