Is Alcée a nice guy or a tool?
That's one of the big questions of the text. It probably depends on what angle you're coming from. Don't automatically place him in the "tool" box just because he cheated on his wife with someone else's, though. Let's put him on trial and examine the evidence.
1. Exhibit A: the fact that he shows up at Calixta's right when the storm begins. Either this is a kind, neighborly way of making sure his old acquaintance is doing OK when the storm hits, or Alcée has some kind of ulterior motive. After all, Calixta is basically his ex-girlfriend. If you think he's being nice, you could observe that he probably didn't realize Bobinôt and Bibi wouldn't be home. When he arrives he tries to preserve good manners and proper behavior, at least for a time: "He expressed an intention to remain outside" for the duration of the storm (2.5). But the storm is too strong and he has to come in. Then he and Calixta are alone in the house together, where the bed almost seems to be inviting them.
2. Exhibit B: he used to try really hard to be honorable and not take advantage of Calixta's virtue. In their previous relationship, he was always the one to stop them before they got carried away physically:
[…] in Assumption he had kissed [her]…to save her he would resort to a desperate flight. If she was not an immaculate dove in those days, she was still inviolate; a passionate creature whose very defenselessness had made her defense, against which his honor forbade him to prevail. (2.18)
In other words, he had to be the one to say no, because she couldn't. When they meet again during the storm, neither of them feels like saying no. Clearly, it's not like Alcée takes advantage of Calixta or attacks her in any way. Their encounter during the storm is totally consensual. They seem to encourage each other. Add to this the idea that Alcée's relationship with Clarisse may not be that great. We know from the section written in her point of view that she doesn't enjoy it that much when they have sex. Well, maybe Alcée doesn't either. Maybe he's just as unfulfilled sexually in his own marriage as Calixta seems to be in hers.
3. Exhibit C: the letter Alcée writes to Clarisse to tell her she doesn't need to come home. Is that evidence of his kindness or of his cunning? Chopin writes that Alcée's composition:
was a loving letter, full of tender solicitude. He told [Clarisse] not to hurry back, but if she and the babies liked it at Biloxi, to stay a month longer. He was getting on nicely; and though he missed them, he was willing to bear the separation a while longer – realizing that their health and pleasure were the first things to be considered. (4.1)
So on the plus side we have the fact that his letter is "loving" and "tender" – he seems to be putting his wife and kids' needs before his own, giving "their health and pleasure" the first priority.
But that last part isn't really true. After all, he's just made his own pleasure a priority. Like Calixta, he found new levels of pleasure in extramarital sex that he'd never experienced with anyone else. After having a really great time with Calixta, maybe he feels like he doesn’t need his wife to come home for a while.
Or maybe he can't stay away from Calixta. Maybe he'll never stop desiring her, and by keeping is wife away he might have another chance to be with Calixta.
So is Alcée good or bad? Maybe he's just like most of us: a complicated human being whose desires don't always match up to what's virtuous, right, or proper. Maybe he should just feel lucky that, for one afternoon at least, he got to have something he'd always wanted.