To prohibit certain sins and acts ALLAH revealed punishments which should be enforced by a state, such as killing someone, committing zina or stealing things. These all acts are punishable by Sharia law which are given by a judge after hearing the case and considering all evidences.
Today there is no country in the world where Islamic system is implemented so where should someone go if he confesses to his crime and is ashamed of his past?
This question was asked to Jamia Binoria on which they responded that today there is no Darul Islam so the person cannot and should not go anywhere for the punishment. Rather he should ask forgiveness from Allah who is merciful to his slaves.
A State where the ruler is Muslim and the political, economical & judicial system is derived from Quran & Sunnah is called an Darul Islam or Khilafah. For more than 1300 years the Khilafah represented Islamic world until the Ottoman Caliphate was demolished in 1924 by Mustafa Kemal Pasha.