I checked but I can't find any errata and/or FAQ on his triggered ability: Dominance: Pay 4 gold to choose and kill a kneeling character. (Limit once per phase.). It says KNEELING and not KNELT... I'm not English or American but the two words are different. It seems to me that Viserion can work only against character that kneel IN dominance (for example a Stark noble character kneeling to play a Guilty! event card...).
Is really correct this way? Because it seems to me a little useless ability (and 4 gold to play it seems quite unbelievable)