Yes, you're instructor is precisely right!
Even in a clean, justified shooting, when the police come have your gun visible (OUT OF YOUR HAND) and be standing with your hands out saying, "good Guy."
They will arrest you, confiscate your weapon and you'll be treated like a criminal. Your case will go before a Grand Jury and hopefully the will file a "clean bill" on your berhalf. You will then be released.
Now, you need to worry about a civil suit from any relatives the perp' had.
Great justice isn't it?