I also was a bit suspicious reading the articles. Here are some possibilities I considered:
"He added that Darcy's estranged husband, Colin Crossman, was always hanging around, and that the pair would go away for weekends and he'd have to pay for it."
Murder trial hears of farmer's love blues
1) Darcy could have been cheating on Dunbar with her ex-husband Crossman. There could still be physical and emotional ties between them, not to mention any children that resulted from their marriage.
"A jury heard Mr Crossman remained married to Darcy and wanted to get a divorce, but he had been bankrupt for three years after his home burnt..."
2) Technically, if Crossman remained legally married to Darcy, he could have stood to benefit had she been the sole beneficiary of Dunbar's estate prior to initiating divorce proceedings.
3) If he was known to be willing to defend her when she tried to actually kill him, then his witness testimony in this trial (and the things he said about Dunbar) could have been him trying to defend her. She could have alluded to him the possibility of another future together, which may create another motivation.
4) If he was bankrupt, he's been able to stay in a home rent free throughout the course of the trial. Not to mention, while alive Dunbar may have also been generous with him.
Of course, I don't know Crossman at all, and he could've totally been altruistic in all of his actions. But it stinks and I'm a skeptic...