Question: How could Anthony Hopkins be arrested for turning off his wife's life support? He was acquitted of shooting her, so he's legally considered innocent of that, and he was completely legally entitled to turn off her life support.
Question: I don't understand why the police haven't checked out Hopkins' gun when he was removed from the crime scene. Wouldn't they check the serial number to see where and when he purchased the gun? The movie mentions that the gun was bought a month prior. Since the gun is actually the officer's gun, wouldn't that registration information come up? Then they would know it was not the murder weapon.
Answer: Crowford (Hopkins) shot his wife with the gun that belonged to Nunally (Burke), but switched it with his at a moment of Nunally's distraction. The gun that the police mistakenly collected was not the murder weapon. This was a key plot point. Crowford goes on to gloat about it near the end of the film, saying the one piece of evidence Nunally needed was on his hip the whole time.
Answer: Since he had meticulously planned every detail, Hopkins would have provided all the necessary information (serial number, date/place of purchase, receipt, insurance, etc.) during discovery. The gun was recovered from the scene, so the police and the prosecution, who believed the case to be open-and-shut, simply didn't check this information against the gun itself and just assumed it was Hopkins'. This is exactly what Hopkins was banking on happening, so the revelation that it the gun was not, in fact, the murder weapon would be a surprise during the trial.
Chosen answer: Once Crawford (Anthony Hopkins) took his wife off life support, it resulted in her dying. Crawford was arrested for murder, not because of taking her off life support but because his shooting of her resulted in her death ultimately. Crawford was only tried (and acquitted) for attempted murder. Since this is a new charge, double jeopardy did not apply.