From the game's encyclopedia
A Kinslayer is a Character who is known to have slain a Family Member. Some Faiths view kinslaying as Criminal.
The three Kinslaying traits are:
• Kinslayer — Given to those who kill Close Family
• Familial Kinslayer — Given to those who kill Extended Family
• Dynastic Kinslayer — Given to those who kill a member of their Dynasty
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Kinslaying is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Kinslayer is a Character who is known to have slain a Family Member." The game also links it as Kinslayer and Kinslayers. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.