Before there was Captain America, or Captain Crunch, or any of those jokers, there was… Captain Arthur Donnithorne. He's the captain of the local militia, a.k.a. "the guy who should be running Hayslope instead of that jerk we've got now."
The present Squire Donnithorne is a disagreeable old man, and when "well-washed, high-bred, white-handed" Arthur comes of age and takes over, he'll usher in better times. For Hayslope's farmers and craftsmen, he outshines every other "captain" on earth "as the planet Jupiter outshines the Milky Way" (5.37).
Arthur's character tells us a lot about the community around him. The truth is, a lot of these people get Arthur wrong. He isn't just a cool-looking herald of better times. He has his faults, especially where women are concerned. Nobody—not Adam, not even wise Mr. Irwine—suspects that Arthur has a weakness for pretty Hetty Sorrel.
So Arthur has flaws, and Hayslope has a flawed way of viewing him. But Adam Bede is all about flaws, all about "inconsistent people whose movements of goodness you should admire" (17.4). What Eliot often shows us is an Arthur that the people of Hayslope don't see. He isn't just "The Captain." And he isn't just the "selfish light-minded scoundrel" who seduces and impregnates Hetty Sorrel (27.29). He's a complex mixture of some bad things, some good things, and a really good hairdo.
Arthur is a nice, easygoing guy—maybe too easygoing:
He was of an impressible nature, and lived a great deal in other people's opinions and feelings concerning himself. (16.38)
He likes flattery. He has trouble standing up to people, including his own irresponsible side. You'll never see Arthur scheming to rule the world, or even see him having one too many at the pub. But he's pretty irresponsible when it comes to sleeping with pretty girls in the time BBC (that's Before Birth Control).
So a guy who really contains about 0% villainy causes more trouble than most villains would. He upends the life of a decent young man (Adam), drives an innocent girl to murder (Hetty), and rattles the nerves of everyone else in the novel. Oops.
The problem is that Arthur's values, good as they are, have never been tested, tried, and toughened. There's Arthur's "native impulse to give truth in return for truth, to meet trust with frank confession" (28.38). Ah, but how often has he had to defend this "impulse" against the temptation to sneak around? Not often. Arthur's values are healthy, in theory. But in practice?
Arthur learns the costs of living in a personal la-la land. And although he's an ace at keeping up appearances, he learns that he can't reverse the past. By the end of the novel, he is once again capable of smiling at Adam "just as he did when he was a lad" (Epilogue.21). Yet carries a sense of loss and grief that won't go away.