I was right behind a truck the other day when it slammed into a deer. The deer was dead when I stopped to check on it and it made me wonder if I could keep the fresh kill.

There is no provision in Idaho law for keeping roadkill and you do not want to be in possession of a dead deer out of season, no tag and so on. If you did keep a carcass, you would most likely wish you had not done so. Collisions with highway traffic usually leave carcasses sadly unusable for human consumption.