Frank Sargent is a tough guy to nail down. On the one hand, he seems like a solid dude—he works hard and can always be relied upon when the going gets tough. On the other, there's no way you can ignore the fact that he hooks up with his best friend's wife. Call us crazy, but that's a big no-no. Because of these contradictions, we'll have to get up close and personal with this home-wrecking hunk to understand where he's coming from.
At first, Frank models himself after Oliver. It's easy to see why: Oliver is basically the GOAT of mine engineering. But, Frank quickly takes this admiration to Stan-like proportions, even growing a mustache simply because Oliver sports one. This actually causes a mustachioed mix-up as Susan mistakenly kisses Frank, "who [...] responded to her embrace with enthusiasm" (4.8.10). Actually, now that we think about it, that might have been the point all along.
See, Frank has been crushing on the lovely Mrs. Ward since the moment he laid eyes on her. We can't blame him for that, but what's truly unexpected is that those feelings are reciprocated—to a lesser degree, at first—by Susan. Of course, part of the reason why Susan falls for Frank is that he reminds her of a younger and hunkier Oliver. But, she also respects his literary knowledge and his dedication to "consciously keeping himself pure [...] as to the awful women he might meet in this place" (4.4.30). And, just like that, the seeds of a torrid love affair are planted.
Susan withstands Frank's advances at first, but her defenses start crumbling around the time her marriage does. Oliver might be a bona fide genius, but he's garbage at running a business, and the family is plunged into hardship while his many harebrained schemes invariably fail. It's no coincidence that this is when Susan gives in to her feelings for Frank. Things get so hot and heavy that Frank even begs her to run away with him for a "disgraceful elopement," as Susan dubs it (8.5.94). Despite these harsh words, however, she seems quite tempted by the idea.
If you're asking us, though, Susan has an overly romantic view of Frank. After all, she fell in love with him in the first place because he was such a nice young man, but look at him now—he's carrying on with his best friend's wife. Not even a congressman could spin those facts in a positive light. In that way, Frank's tragic suicide suggests that he has realized that he has strayed from his morals—and that he's convinced himself that he must pay the price. The punishment is way worse than the crime in this instance, but real life rarely functions as neatly as an episode of Judge Judy.