One of the major subplots of House of the Dragon’s first season is Ser Criston Cole switching sides from the Blacks to the Greens (well, technically), incited by Rhaenyra’s rejection of his offer of moving to Essos and having a married life.
Cole’s resentment towards his former lover makes him commit some heinous acts throughout the first season, one of them being the murder of Ser Joffrey Lonmouth (Solly McLeod). Ser Criston punches him until his face turns to pulp, much to the horror of the audience that is gathered in the hall where the incident takes place. That he does it during Rhaenyra’s wedding celebrations to Ser Laenor speaks volumes about his pettiness and immaturity, which are only compounded as the series goes on.
Cole commits the act after Ser Joffrey suggests a polyamorous arrangement between the two of them and the newlywed couple. For context, Rhaenyra and Ser Laenor are secretly entering a lavender marriage, as the latter is in a liaison with Ser Joffrey while Rhaenyra had offered Ser Criston a relationship out of wedlock. However, Ser Criston doesn’t accept the arrangement and instead becomes spiteful, ashamed at himself for being turned down and then being presented with the idea of being a “whore,” in his own words.
Ser Joffrey’s suggestion tips Ser Criston over the brink, and he ends up taking the life of Ser Laenor’s paramour. While all signs point toward Ser Criston’s fragile ego being the reason for Ser Joffrey’s death, another potential explanation could be that the former was scared about more people finding out about his escapades with the princess.
The Kingsguard are sworn to a vow of celibacy, and to break that vow with a princess (who is assumed to be a virgin until her marriage) would have resulted in an unkind demise for Ser Criston, to put it politely. Hence, it is possible that the now-maligned knight did it in self-defense, more than anything else.
