The Bolsheviks?

The Bolsheviks were scared to death of political prisoners who supported the Czar and had thousands killed including the Czar and his family. There was a Civil War in Russin from 1918 to 1922 between various supporters of the old way and the Bolsheviks. As for a constitutional monarchy, they would never have even considered that because the did not believe in either democracy or the divine right of kings. You'll find a lot of info on wikipedia if you wants specifics on Lenin and the Bolzheviks, the imprisonment and exectution of the Last Tsar as well as the Civil War and its causes.