Mr. Murdstone is clearly the enemy of David's early days. Mr. Murdstone horns in on David's happy, peaceful relationship with his mother. He humiliates David until he feels stupid, lonely, and isolated from the world. He whips David and keeps him locked in a room for five days until he can send David to a vicious school, Salem House. And then, when David's mother dies, he sends David to a factory to work as a drudge with no money and no future. So yeah, we think it's fair to say that Mr. Murdstone is David's antagonist. But once Miss Betsey tells off Mr. Murdstone in Chapter 14, Mr. Murdstone's power over David is utterly broken. Miss Betsey is David's new guardian. So, we have to look to other characters to find new sources of conflict for our hero, David.
Uriah Heep gets his claws into nearly everything in this book: he has designs on David's own future honey, Agnes. He slowly destroys Mr. Wickfield's health and reputation by encouraging Mr. Wickfield's weakness and alcoholism. He's the one who tells Doctor Strong the whole world thinks Annie is cheating on him. And we find out in the end that it was Uriah Heep who hid Miss Betsey's money from her, making her believe that she is financially ruined. So, Uriah Heep is the primary enemy of roughly five-sixths of the book, once Mr. Murdstone is out of the way. Even in the early chapters of their acquaintance, David is so disturbed by the clammy feel of Uriah Heep shaking his hand that he actually wipes his hand off, "afterwards to warm it, and to rub his off" (15.73). This is definitely a bad sign for Uriah Heep's antagonistic relationship to David.