The first thing we find out about Farmer Boldwood is that he's a really dignified guy. Or, as the author tells us, "He was erect in attitude, and quiet in demeanour. One characteristic pre-eminently marked him: dignity" (12.14). Also, the guy is loaded. He's 40, but many of the women in Weatherbury still pursue him pretty aggressively, hoping that he'll propose to them. But none of them ever have any success until Bathsheba sends him a Valentine saying, "Marry Me" on it. The problem is that Bathsheba never wanted him to find out who sent it.
And now we find out about a deeper layer to Boldwood's character. In the words of the narrator, "If an emotion possessed him at all, it ruled him: a feeling not mastering him was entirely latent" (18.6). Or to put it another way, the guy becomes totally obsessed with his love for Bathsheba. He's so obsessed with possessing her, in fact, that he doesn't even care whether she loves him. He just wants her to be his wife.
Now as you can imagine, it's hard to go from being a guy who's always been in control of his image and his behavior his entire life, and to be reduced to a little schoolboy with a crush. But that's exactly what people notice about him the moment he gets rejected by Bathsheba: "There was a change in Boldwood's exterior from its former impassableness: and his face showed that he was now living outside his defences for the first time, and with a fearful sense of exposure" (18.12). The poor guy doesn't like being vulnerable in this kind of situation, and this feeling of vulnerability eventually causes him to lash out and murder Sergeant Troy, effectively ending his own life, too.