If its illegal, then you shouldnt be doing it. Plain and Simple!
Keeping it and not realeasing it.. pffttt.. If it threaten your status/reputation, even know you know its a wrong doing, most of you will end up doing it..
Let say, you own a 12" snakehead in this wonderful 200 Gallon tank in a nice beautiful home in CA, and your backyard faces a river.. For years its been peaceful.. One day, your neighbor come and ask for something, and he noticed that you own a snake head. Then your neighbor said he will report you.. Later that day, some officers come by.. (Knocks on your door) What will you do? Get the fine and get charge, and risk of going to jail? Release the fish and pretend nothing happen? ^^
Some of you said, I want to keep an illegal specie, as long as I dont release it, but what if this scenerio apply to you? What will you do? Id say it will probably end up in the river! ^^