For the right age, Assassin's Creed III can help with learning about and creating interested in history and moral decisions. If you play the game, and follow the story closely, you'll notice that the Assassin's, as a group, are not blood thirsty killers, but people who try to preserve the freedom of others. Does Conner kill people in this game to achieve this end? Yes. But so did our founding fathers.
I also find it interesting how much people focus on the violence, when along with it, you have the option to help others by taking them to the doctor when they've contracted small pox. You also have the option to burn the blankets, pay mortgages so people don't lose their houses, collect pieces of the Pennsylvania Gazette, sell and buy items to vendors, defend citizens from abusive soldiers, and so much more that could help teenagers look at real problems, and show how good it is to stop people like that.
Is it bloody? Yes. Is there a little swearing? Yes (I don't remember a single F-word though). But is Conner really not a possible role-model just because he kills people for a greater good? Just like my friends and family in our military, I'd say no.