Confidence can be a good thing. It can help motivate you to achieve whatever goal you are trying to reach. A positive attitude is the only way that you should look at things. However, being overconfident can actually be someone's undoing. This is especially true if that person is a police officer. Their overconfidence can be seen in their attitude and can turn a jury against them.
People expect police officers to be responsible, unbiased figures of authority who are doing their jobs in a civil manner. When these people see a police officer act in a cocky or arrogant way, their respect for that officer starts to decrease. The defendant starts to become a victim in the eyes of the jury, and the officer starts to resemble a bully. The confidence that allowed the police officer to think he was invincible has actually led to him being at a disadvantage during the case.
This attitude can manifest itself in a number of ways during the trial. The way the officer describes what happened is a main one. Being demeaning to the defendant by acting superior will not impress the jury, but will in actuality harm the image of the officer. The simple way that the officer composes himself can be taken as being overconfident in a damaging way. If the officer thinks he is better than everyone else, that includes the jury, who will then want to punish the officer out of pure dislike for him. Confidence can be a great motivator, but it should be known that it can be destructive just as easily.