The murder rate was declining both long term and in recent years, here is the annual homicide rate per 100,000 for the Trump years:
2016 5.4
2017 5.3
2018 5.0
2019 5.0
And in 2020, the rate was flat at the beginning of the year, even through the initial Covid lockdown. It only started spiking in June. And not just a spike, likely the largest increase in the homicide rate in US history.
The author, by ruling out the protests as a cause of the increase in the number of people murdered, seems to make the implicit claim that protests can't cause political instability, that they can't decrease trust in government or "People’s faith in political institutions". But no evidence is presented for this implicit claim.
There was so much focus on police killings last year that it seems to have cause people to be misinformed. In one survey, 44% of liberals and 20% of conservatives estimated the number of unarmed blacks killed each year by police to be over 1,000. That's two orders of magnitude higher than the actual number. How can that not have an effect on people's views of "the legitimacy of political institutions"?
In any event, if you are going to make the case that "x" caused the massive recent spike in murder, you need to explain why "x" had an unprecedented spike starting in June of last year.