Other mistake: After killing his inmate, notice the blood on the inmate's neck is present in great amount. After camera changes, the blood is only on the shirt, the neck is clean with no holes nor coagulated blood. (00:51:00 - 00:51:45)
Other mistake: When Clyde is in the van driving back to the prison he encounters a road block. He evades this by driving down into the parking garage. There's no way that the police or SWAT would have missed this loophole. (01:32:34)
Answer: Because Nick knows this man is guilty. He says it to almost imply, "well since I'm getting off, may as well make a visit to your family next. You married, Nick?"