Also seen that they could be made in reference of the seven deadly sins, which I can get behind.
Nihilego: Envy, Nihilego is a parasite, taking control of it's victims, this can relate to envy in the sense Nihilego is envious of it's victims and pretends to be them.
Buzzwole: Pride, Buzzwole is constantly described as boasting it's musculature and strength, taking pride in itself.
Pheromosa: Lust, Pheromosa is said to attract Pokemon regardless of their gender, making other Pokemon lust after it, but dumbed down for the PG.
Xurkitree: Greed, it's moon dex entry says it raided a power plant resulting in people thinking it energizes itself with electricity, so it's greedy for electricity.
Celesteela: Wrath, it's moon dex entry says it was witnessed burning down an entire forest by expelling gas from it's arms, expelling wrath on the land.
Kartana: Sloth, it's sun dex entry says it doesn't attack on it's own but it's sharp body acts as a weapon, it doesn't care to attack so it's lazy in that sense, since it's sharp body will do the job.
Guzzlord: Glutton, the most obvious one, it's UB name even gives it to us! UB-Glutton. It's dex entries say it eats anything and everything and doesn't even leave waste behind.