We discuss Alec before Angel not because he's more important to Tess, but because he's more important to Tess of the D'Urbervilles, as a novel. His actions drive the tragedy forward, and his obsession with Tess – both at Trantridge, and then later, at Sandbourne and around the north of Wessex – bookends the novel.
Alec is clearly the bad guy in this novel – after all, he rapes our heroine and pretty much ruins her life. And yet it's hard to read him as wholly evil. How does Hardy manage to pull that off? Alec changes as the novel progresses. He's not stagnant as a character, which gives him depth. He's not a one-sided villain. But how much does he change? What's his "character arc" like, and why does Hardy shape him the way he does? Let's start at the beginning, with the initial description of Alec at Trantridge.
He had an almost swarthy complexion, with full lips, badly moulded, though red and smooth, above which was a well-groomed black moustache with curled points, though his age could not be more than three-or four-and-twenty. Despite the touches of barbarism in his contours, there was a singular force in the gentleman's face, and in his bold rolling eye. (5.29)
There's some subtle racism in this description – his "swarthy complexion," "full lips," and "barbarism" call to mind some of the racial stereotypes spread by people during the Victorian period who wanted to justify England's colonial occupation of various countries around the world. And part of the stereotype (applied pretty equally to almost any non-white, non-British individual) was that those with darker ("swarthy") complexions were more lusty and sexual than folks from colder climates like Great Britain. So this opening description of Alec uses racial stereotypes to suggest that he is more interested in the physical than the spiritual side of love, and that he's very lusty and sexual.
The other interesting thing about this description is the mustache. Earlier drafts of Tess of the D'Urbervilles make Alec look younger, and more boyish. But in the final version, Hardy decided that Tess's seducer/rapist should be more adult. Unlike Tess, he knows what he's up to, and what the consequences will be, and he does it anyway. Also, a "well-groomed black mustache"? It's like he's a stage villain – Hardy whips out every stereotype in the stereotype book to suggest to us Alec's out-of-control sexuality, and his role as a villain.
But despite the rampant stereotypes in the initial description of Alec, he still proves capable of change. After publicly humiliating his would-be savior, the Reverend Clare, Alec eventually converts to Christianity, and starts preaching on rocks and in village centers.
But did he really change? When he sees Tess again, his newfound faith waivers, and he chases her down, asking about her life since she left Trantridge. He even offers to make amends for the past as best he can by proposing to marry her and make an "honest woman" out of her. When she turns him down (she's already married, after all), he gives up on being a preacher and turns into the same old mustachioed, obsessed seducer he was at Trantridge.
So was his conversion sincere, at least while it lasted? The trouble with Alec is that he's extremely susceptible to passing impressions – Tess's beauty is all-absorbing, but he forgets her after she leaves Trantridge. His conversion works the same way – as long the impression lasts, it's all-absorbing, but it's not deeply felt enough to last any longer. As soon as a new impression comes along –Tess, again – he drops the Christianity and becomes obsessed with Tess.
Alec's conversion doesn't necessarily make us like him any more. It might even make us like him less, since it shows how easily persuaded he is by any passing impression. But it does make his character more complicated. The conversion episode allows Hardy to give his villain more apparent depth.