Interesting idea... of course, there would have to be some lower cutoff, else if I were judged to have committed a murder with 20% probability, I would have to serve 5 years. There might be a place for these probabilities, but I suspect it would be in the appeals process or some such.

You (hypothetical person) are unnatractive, that raises the probability that you will commit murder or rape. You are hereby convicted and sentenced to p% of death.