A penalty lower by one or two degrees than that prescribed by law shall be imposed if the deed is not wholly excusable by reason of the lack of some of the conditions required to justify the same or to exempt from criminal liability in the several cases mentioned in articles 11 and 12, provided that the majority of such conditions be present. The courts shall impose the penalty in the period which may be deemed proper, in view of the number and nature of the conditions of exemption present or lacking.