The general purposes of this title are:

(1) To forbid and prevent conduct which unjustifiably and inexcusably causes or threatens substantial harm to individual or public interests;

(2) To give fair warning of the nature of the conduct forbidden and the sentence authorized upon conviction;

(3) To define that which constitutes each crime; and

(4) To prescribe penalties which are proportionate to the seriousness of crimes and which permit recognition of differences in rehabilitation possibilities among individual criminals.