No awards for the most upstanding person, as he was cheating with Amanda, but it is still hard to believe he would propose to Meghan and then murder her the same night.
Although Amanda did harbour an unhealthy level of resentment of Meghan and spoke if it to Dirk, he had little motive to commit the crime.

Backstory...
Originally from a blue-collar neighbourhood in Manchester and raised mainly by his father, James.
James was an iron worker since high school with the local Associated Iron and Steel Workers Union and not around much once his wife of 12 years and elementary school teacher, Emily, died from cancer when Dirk was 10.

The one, and maybe only, thing he shared with his father was a passion for working on cars. When not working or at the pub, James could always be found in the garage working on his prized 1967 Triumph Herald. Not highly collectable, but extremely nostalgic for him, he would find some joy in teaching Dirk about it and how to maintain it. Somehow focusing on the car rather than the mundane nature of their reality, made for a pleasant diversion for both. Those were the fondest memories for Dirk.

Otherwise, as a result of little stewarding from any significant adult role model, Dirk was left to develop a street savvy, hustler’s mentality, despite completing one year at MCC (Manchester City College). As most of the kids did growing up, Dirk played football, and was actually a solid striker. But unfortunately, the lack of discipline and support at home, along with the tremendous competition in general, made it difficult for Dirk to maintain the focus and progress towards any professional aspirations. He would still find occasion for a pick-up match at the local pitch to keep in shape.

With the option of professional footballer, albeit slight, gone, he worked the club scene and managed to pick up as a bartender at the Armory in Liverpool. A tough task for a guy from the streets of Manchester, but step closer to something better. His street senses, quick wit and rugged looks were perfect allies in the social cesspool of the club and pub scene, where one could target and engage a trust fund socialite, promising VP or even local politician for financial or social gains. This is how he met Dalton O’Neal, owner of The Commons Pub in London, which resulted in his position as lead bartender there.

*Unwittingly had been poisoning Meghan with ethylene glycol through they laced kumquats Amanda was supplying, but not what caused her death.