This verse succinctly states why it is so important not to hold the spirit of murder within us. Anger or resentment may flash into our minds, and we have not yet sinned. But if we hold it and allow it to burn, it could very well destroy us!
Hatred also is the spirit of murder. But beware! Human nature can lead us into thinking that hatred has no serious, immediate consequences because the Lake of Fire seems so far off. The spirit of murder must be nipped in the bud before it leads to murder or the Lake of Fire. Notice Matthew 5:23-24, the verses immediately after Jesus' statement on the spirit of murder:
Therefore if you bring your gift to the altar, and there remember that your brother has something against you, leave your gift there before the altar, and go your way. First be reconciled to your brother, and then come and offer your gift.
Do not attempt to make any offering to God while in the spirit of enmity! Jesus' words clearly imply that God will not accept our worship while we hate another person! Can we honestly say we are worshipping God in spirit and truth when we hate a brother? How can a heart burdened by grudges offer God complete adoration? Within God's court there are no unsolved crimes, nor does He lack the power to see our inner motives.