If untreated is syphilis deadly?

Absolutely. The Tuskeegee study, in which mostly black men in the US south known to have syphilis were followed until they became symptomatic with the advanced disease rather than being treated, was a crime. However, it left no doubt that one can indeed die of syphilis. Even today, one occasionally sees neurosyphilis or a ruptured thoracic aneurysm.
Can be. Syphillis can go into a relatively dormant stage in untreated. Over time it may attack specializes structures like the heart and brain with deadly results. It has been proposed that hitler showed signs of neurosyphillis in his final years and well known a gangster died in prison of its effects.Tissues of either structure cannot heal after scarring.