a better thing for Doc to do would be come back at the end of Part I and say, "Marty, your kids turn out to be assholes. In 30 years, make sure you keep an eye on them." Takes less jiggawatts that way.
Doc probably thought that it would be better for Marty to see the future firsthand, to kind of shock him into changing his ways. Doc may have even known about Marty's tendency to freak out when somebody calls him "chicken". As Marty's friend, Doc thought this would be the best way to help him - by showing him what the future would be like. Otherwise Marty might have just sort of shrugged it off.