The idea that you're forgiving of actual criminals (up to and including murderers) but not someone who litters, is actually mental.
But you know that, right?
RIGHT?
Murder, that is, the killing of an individual, may be justified - for right or wrong - by a motive, whether personal or political. Exactly as it is in the case of war, murder is a very complex topic which is not only beyond the scope of this thread, but also forgivable - if not necessarily excusable - for any number of reasons.
A man may kill another based on culture, tradition, honor, political passion, or even percieved moral conviction; but the litterer exhibits no such passion. To the contrary, he proves that he has no desire to respect or honor himself, his fellow, or any common rule of order. His virtue or vice is not rooted in strength of character, but in lack thereof; and unlike many evils which the doers thereof hide beneath the surface of their lives, that small vice of littering impacts not merely the infected party, but all those beyond the realm of its grasp.
Litter reflects a psychological condition of low self-esteem; it fosters a condition of moral depravity. It is thus no surprise that the regions wherein one may find the greatest degree of criminal activity and human despondancy are also rife with unkept yards and filthy streets.