Well unfortunately, if you bonk a fish that is illegal to keep you have to suffer the consequences. When we go out and fish we have a responsibility to insure we are following the regulations for the stream or lake we are fishing on. That would mean at the very least, I hope reading the regulations, and being able to identify the different fish species. So even though this person who killed the steelhead did so by mistake he still committed an offence.
Now if this individual had called Fish and Wildlife, and called himself in .....maybe the Conservation Officer would have been lenient on him.
But by running away, it is my understanding that he would face a fine, seizure of fishing gear used to catch the fish, and possible suspension of his fishing license.