Posthumus isn't your average knight in shining armor. He was taken in and raised by Cymbeline as a kid after his parents died (hence his name: he was a "posthumous" baby), and then he married Imogen right under Cymbeline's nose. Some way to say thank you, right? People back in those days didn't go for that kind of thing, even if true love was involved.
As for true love, we know that Posthumus totally loves Imogen by the way of two of them talk to one another in their first scene together: there's lots of "I love you"—"No, I love you more," and gushy stuff like that. He calls her his "queen" (1.1.107) and promises to be "the loyal'st husband that did e'er plight troth" (1.1.111). It's all very gushy and sweet. He loves her. She loves him. They'll be together forever. You know the drill.
Too bad that doesn't last beyond the first act. After arriving in Rome, Posthumus quickly enters into a bet with the tricky Iachimo. Iachimo's all: "I bet I can get it on with your wife behind your back." And then Posthumus is all: "I BET YOU CAN'T." Seriously, this dude is way too eager to prove his wife's chastity.
Okay, okay, we know Iachimo is scheming and manipulative and all that jazz. Just as the Queen manipulates Cymbeline, Iachimo is pretty good at manipulating basically everyone around him—but especially Posthumus. What we want to know is: why is Posthumus so quick to offer up his gal as bait?
On the one hand, he's totally convinced she'll be faithful to him. Some critics say that it's actually his confidence in her that motivates his actions. That's a nice way of looking at it: according to these fancy scholars, Posthumus knows she won't stray, so he's willing to risk it all.
But is it worth it? Even if he trusts his wife 100%, why bother making a bet about it? Why offer her up to Iachimo on a silver platter?
Now, there are others who that think Posthumus is actually insecure about his wife's honesty, and maybe that's something he wonders about while he's so far away. He can't actually see what his wife is doing, so who knows—maybe she's actually not as honest as he thinks. If so, then Iachimo can see this insecurity a mile away, and he knows that's a button to push for Posthumus. That's why there's the bet in the first place.
Whatever his motivations for betting, Posthumus quickly learns the hard way that no good comes from bets in literature: someone always ends up fooled or cheated, or both. Basically, Posthumus walks right into Iachimo's lies. He wholeheartedly believes his wife has done the nasty with Iachimo and wants her to pay for it. When he goes into conniptions about how vile women are, it's a pretty clear message to us that he's actually been hurt by the whole scam.
That might not excuse putting a hit out on your wife, but still.
Eventually, Posthumus comes to regret his actions. His saving grace is that he realizes he was wrong for ordering Pisanio to murder his wife (duh), and he tries to make up for it. He tells us he's "more worth [Imogen's] vengeance" than love (5.1.11) and that he is "merrier to die" than live (5.4.175). This guy can be a real downer, but we'd say he's earned his moping rights.
It's almost like he needs to feel really, really bad about his actions before things can get better for him, as if there's a lesson he's supposed to learn before he can get on with his life. Like Cymbeline, another dude who's easily led astray, he has to face the consequences of his actions before anything can get better. Of course, once he learns the truth, he's in a much better place: he's willing to let Iachimo off easy as long as he has Imogen back.
Sure, he saves the day with the help of his brother-in-laws, but the real focus of Posthumus's story is his love for Imogen. In the end, the two of them reunite, they're finally allowed to live happily ever after. Maybe they'll even live more happily now than they would have—now that, you know, their love has been tested and has survived the test.