Answer:
Emperor Nero:
In the first few centuries AD, a lot of Christians used to get killed and prosecuted, punished by death penalty, but there were also times when they were really secure. However, Christians were first persecuted on a large scale under the rule of Nero.
Nero had a plan to tear down the one third of the city of Rome, just so he could build his own series of palaces there, The senate objected to this proposal.
In 64 AD, a fire broke out and burnt most of the Rome. When the fingers started to point toward Nero, Nero used Christianity to cover an act of his person hunger. He blamed Christians for the fire and ordered that Christians should be rounded up and killed. A mass murder of Christians was conducted by Nero, where Christian were torn apart by dogs and burnt alive.
Emperor Constantine:
In the era of Constantine, Christianity started to become a dominant religion. People at that time couldn't understand why Constantine used to favor Christians. One of the reasons was that his mother was also a Christians.
However, his main objective behind this scheme was to get unanimous approval from all major religions and classes of the Roman Empire. As Christianity was a major religion of the Roman Empire, he chose to use it for his political agenda.