He looked familiar to me also, so I looked him up. He was the prince in those Netflix royal Christmas movies that were so popular.
I could be wrong, but I think the elder Cresswell was the intended target to begin with. Bell's wife was accused of having an affair with him. I think we were to assume that Farmer Bell killed his wife and went after the elder Cresswell before then killing himself. Then, whats-his-name, who had helped with the poison pen letters by mailing them, committed suicide due to guilt. So the vet's daughter only directly killed one person, the older Cresswell brother.
It's interesting that people find this season to be so dark. I guess the personal stuff with all of our main characters is very dark, moreso than in the past. But I think there's actually fewer direct murders (so far) than in previous seasons. More accidental deaths. A lot of the murders in the past seasons were super dark, or the stories behind them were super dark (like the storyline with the prostitute suspect that someone else referenced here). This show has never been rainbows and butterflies, though, lol. I still enjoy it as just a quality show. And I've always had some trouble following the storylines, especially if I don't pay 100% attention. It helps using closed captioning, though. I miss some things with the accents.