Here is a good explanation:
Q. 135. What are the duties required in the sixth commandment?
A. The duties required in the sixth commandment are all careful studies, and lawful endeavors, to preserve the life of ourselves
721 and others
722 by resisting all thoughts and purposes,
723 subduing all passions,
724 and avoiding all occasions,
725 temptations,
726 and practices, which tend to the unjust taking away the life of any;
727 by just defence thereof against violence,
728 patient bearing of the hand of God,
729 quietness of mind,
730 cheerfulness of spirit;
731 a sober use of meat,
732 drink,
733 physic,
734 sleep,
735 labour,
736 and recreations;
737 by charitable thoughts,
738 love,
739 compassion,
740 meekness, gentleness, kindness;
741 peaceable,
742 mild and courteous speeches and behaviour;
743 forbearance, readiness to be reconciled, patient bearing and forgiving of injuries, and requiting good for evil;
744 comforting and succouring the distressed and protecting and defending the innocent.
745
Q. 136. What are the sins forbidden in the sixth commandment?
A. The sins forbidden in the sixth commandment are, all taking away the life of ourselves,
746 or of others,
747 except in case of public justice,
748 lawful war,
749 or necessary defence;
750 the neglecting or withdrawing the lawful and necessary means of preservation of life;
751 sinful anger,
752 hatred,
753 envy,
754 desire of revenge;
755 all excessive passions,
756 distracting cares;
757 immoderate use of meat, drink,
758 labor,
759 and recreations;
760 provoking words,
761 oppression,
762 quarreling,
763 striking, wounding,
764 and whatsoever else tends to the destruction of the life of any.
765