The police are presented as being very professional and intelligent. The manner in which they speak is very formal.
Dewey mentions that his attitude towards the crime was a "Personal proposition" because his family knew the Clutters. This shows that he has deep morals and wants to bring justice to his friend.
He also mentions that even if he didn't know the Clutters, he would still treat it the same because it was horrific, and unexpected.
In Dewey's dream about Perry and Dick, Capote presents them as running away and Dewey not being able to catch them, this symbolises Dewey's frustration at not being able to bring justice to the Clutters fast enough.
Nye's ability to corner the convicts is proven when he is interrogating Dick. He made Dick believe that he was deceiving him, made him feel comfortable, and then at the last minute asks "hotly" "Why have you been lying to us?" This is climatic and shows that the police have already gathered information, which then proves their abilities to work as a whole force and that they know what they are doing.
Compared to "True History of the Kelly Gang", the police and law are presented as bad guys, as we are seeing them through Ned's eyes. Although some of Ned's intentions may not be legal, he still has a good thought behind them connected to his family,
The police are perceived as quite dumb, because Ned outwits them many times. However, the police force and government are quite corrupted and have many double standards. Examples of prejudice toward the Kelly family can be seen when they are referred to as "irish thieves"
The police in KG seem very rude and abrupt too, in their manner of speaking. For example, they use a lot of slang. "if you duff another fellows heifer" shows lack of professionalism.