IMO, when it comes to 'injustice collectors,' there's really no rational reason for their actions since most of the hostility is built around perceived wrongs-- versus what most would deem real and/or actual wrongs. McKee may have been triggered by seeing a Hallmark holiday special, and angry that his ex-wife appeared to be living a Hallmark life during that fatal week.
For example, I just watched a YT series this week for the ten year anniversary of the 2015 DC Mansion Murders, by Mike King (Profiling Evil). In this case, Daron Wint, the convicted killer, had been terminated from the family's construction firm 10 years earlier.
While some suggested that Wint had been flagged for deportation in 2015, and retaliated against Savvas Savopoulos, Wint, a certified, union welder, certainly could have found employment elsewhere between 2005-2015, or the 10 years since he was terminated from American Iron Works! Totally irrational motive. JMO