Romans didnt kill Jesus Christ, the Jews did.
Romans invaded Palestine and Judea and hated the jews, they massacred them at various occasions. killing the first borns etc. Jesus was about to establish a new religion. and that was a thorne in the eyes of the Jewish priests. Most Romans didnt even know about jesus existence. The jews (in the form of judas) betrayed jesus, the jews persecuted jesus, and also judged uppon him, they said that jesus is the king of the jews and plans a revolt agianst rome(which is a total lie, jesus is the son of god, and did not plan a revolt against rome). Because pontius Piltes famous words "I WASH MY HANDS IN INNOCENT" , but the romans were the rulers of this land and there for the justice system. so the romans had to crucify him. under the terms that jesus was a troublemaker. Petrus tan FLED to rome. fearing persecution by he jewish priests. He converted a bunch of roman civilians. to christianity. Christianity grew rapidly across rome. because it promised them a life after death. It became a thread to the roman leaders, because the roman beliefs were in multiple gods(as we all know Mars=god of war, Jupiter=god of all gods, Fortuna=godess of luck,etc.etc.)
so the Roman Christians were forced into the underground, literrly, they builded CATACOMBS and had secret messages. Nero, persecuted the roman christians, and threw the captured ones in the Colosseum against the lions. But Caesar Constantine, had a dream about a Fire Cross and an angel talking to him, saying "if you put a cross on each legionare, than, you will win the upcoming battle". So he did, and so he won. and made christianity the official religion. and christianity spread across the whole roman empire (todays europe).and the Roman Catholic church took shape.
Roman Catacombs
http://www.aug.edu/augusta/iconography/catacombs/orpheus.jpg
http://www1.appstate.edu/~gravettsl/RomeCatacombs.jpe
http://classic.archives.nd.edu/maritain/im...ce/Catacomb.jpg