There are a few bad seeds in any group of people. Most doctors are smart and caring, though there are a few that actually harm patients on purpose. That doesn't mean that they are all bad. This goes the same for the police. Sure, there are a few dishonest, unlawful officers, but for the most part, they are in uniform to serve and protect. We just don't always see the good things they do, as the media only tends to report on the bad.
No, officers cannot be believed in this case. The video proof is showing evidences against him. At court, the video proof is easily more believable than a person's testimony, unless proven otherwise. The lawyer may try to scrutinize for any discrepancies happening in the video like timestamp, sudden change in background, lighting, etc. but this is hard to do and the rebuttals may not be substantial.
Unfortunately, our law enforcement officers do not always follow the law when it comes to gather criminal evidence for prosecution. Some police will fabricate charges against suspects. Therefore, we cannot always believe was police officers tell us. Police must be held accountable with body cameras, and checks and balances to make sure that a suspects rights are not violated.
No, officers cannot be believed when the video shows contradictions to their stories. This is the reason police must wear body cameras. They often try to bend the truth in order to get away with a crime, and they must be watched closely to ensure they do not take innocent lives.