In most cases people who commit suicide are in ill mind (clinical depression, psychosis, and so on), so I don't think calling it 'sinful' is such a prudent choice.
I'm not wholeheartedly endorsing the traditional view, which obviously predates modern psychology, just pointing out that there is a rationality to it that continues to be valid today. People make mistakes, and people are judged for those mistakes, and sometimes people make amends. The social fabric heals. Suicide takes all that off the table, makes people guilty of murder in the eyes of others, and leaves the survivors with precious little way to heal the social fabric.
