Definitely seen excessive use of force against seemingly peaceful protesters, but context is everything, and a 10 second snippet does not tell the full story of the interactions between that protester and that cop for the 45 minutes leading up to those 10 seconds.
I tend to reserve my judgement on those types of videos, since cops have been also taking a beating in the last few days with bricks, rocks, water bottles, etc... thrown at them off camera or before those clips start, where the convenient "editing" is done to portray the cops as the "bad guys" when often they have spent the last hour being insulted, assaulted and injured before they decide to move in with force on that "peaceful" protester...
Also who gives a shit if they just got verbally harassed for an hour. I don’t get to lose my shit and delete a project because someone keeps submitting bad code. Why do cops get to lose their temper when their job is to keep the peace and enforce the law. Learn to control yourself and do your job or find another profession.
TITLE 18, U.S.C., SECTION 242 DEPRIVATION OF RIGHTS UNDER COLOR OF LAW
Acts under “color of any law” include acts not only done by federal, state, or local officials within the bounds or limits of their lawful authority, but also acts done without and beyond the bounds of their lawful authority; provided that, in order for unlawful acts of any official to be done under “color of any law,” the unlawful acts must be done while such official is purporting or pretending to act in the performance of his/her official duties. This definition includes, in addition to law enforcement officials, individuals such as Mayors, Council persons, Judges, Nursing Home Proprietors, Security Guards, etc., persons who are bound by laws, statutes ordinances, or customs.
This means Chauvin could say he was acting under the "color of law" when he attempted to restrain George Floyd and during his restraint is when Floyd died. Because Chauvin was acting as a police officer and not a legal citizen, it was within his official duty as a police officer to apprehend someone who is breaking the law.
However, in the initial part of the statue, it clearly states:
This law further prohibits a person acting under color of law, statute, ordinance, regulation or custom to willfully subject or cause to be subjected any person to different punishments, pains, or penalties, than those prescribed for punishment of citizens on account of such person being an alien or by reason of his/her color or race.
This also means the prosecution can simply say that Chauvin was a known racist and violent around black people and his restraint was not an approved technique for restraining or arresting someone, therefore, based on those facts, the prosecution can make a compelling case that Chauvin attempted to use "color of law" to cover his willful infliction and in doing so, deprived George Floyd of his civil rights while doing so.
The shorter answer is yes, they can lose their temper because it might be required in the application of their duties as a police officer and officer of the government for which they serve. No, in that they cannot abuse that power and use "color of law" to deprive someone of their civil rights under the constitution.