It's best not to release animals that have been kept with non-natives for any length of time. You never know what they may be a asymptomatic carriers of - you could release a new disease or parasite into your area.
Here in CA, it's illegal to release native desert tortoises that have been in captivity due to their potential to infect the wild population. Just re-home the turtle if you don't want it anymore. Map turtles are popular, shouldn't be too hard to find somebody who wants it.