[1 John] 3:15 Hates...murderer. Cf. Mt 5:21-22.
[1 John] 3:19-20 By this, by loving in deed and truth (18) and so having God's love abiding in us (17), we shall know that the truth which sets men free from sin and self (cf. Jn 8:32) has sway over us and determines our actions. Then the accusation of our consciences (hearts, 19, 20) will fall silent before the mightier testimony of our deeds and, much more, before the mightier testimony of our God, whose love lives in our hearts. In that love He is mightier (greater) than our faltering human hearts; that love is the awesome love of the God who forgives (Ps. 130:4), the love of Him who says, "I will not execute my fierce anger...FOR I am God and not man, the Holy One in your midst" (Hos 11:9), the love whose measure our hearts cannot take (Ro 5:6-8).
[1 John] 3:20 He knows everything. Cf. Mt 25:31-45, where the Son of Man, as divine Judge, knows and values deeds of love which those who did them would not dare to plead in their own behalf.
[1 John] 3:24 The Spirit. The mention of the Spirit serves a twofold function. It makes clear that our assured consciousness of God's love in our lives is not our own subjective mood-making but God's own work; and it provides a transition to 4:1-6, with its antithesis between "the Spirit of God and...the spirit of antichrist." (2, 3)
Martin Franzmann and Walter H. Roehrs, Concordia Self-study Commentary [commentary on 1 John], p. 275