# who said about his father and mother
The word "who" refers to the Yahweh's "faithful one" of Deuteronomy 33:8. This is the tribe of Levi.
# your word
"your commands"
# guarded ... kept
These words often refer to "obeyed," but here they should be translated literally because they refer to a time when the Levites literally killed people who were rebelling against Yahweh.