murder one's lord or father
弑 centers on the grave act of murdering one's own lord or father, a specific form of killing that violates a sacred bond of loyalty or filial piety.
弑 combines 杀 (kill) with 式, which likely contributes a phonetic element. The character specifically denotes killing a superior, a concept distinguished from general murder.
The left side 杀 means kill, and the right side 式 suggests a formal pattern. Imagine a ritualized killing of a lord or father, following a deadly script.
For シ, picture a scene of betrayal: a servant she (シ) murders her lord. The sound 'she' cues シ while the act fits the kanji's dark meaning.