Christianity becomes a persecuting religion

In the view of historians (for example Coffey), the shift implemented by Constantine turned Christianity from a persecuted into a persecuting religion. (See John Coffey, Persecution and Toleration in Protestant England 1558-1689 )
  
Examples of Christian violence over the last 2000 years include the Inquisition, the Crusades, the Wars of Religion, antisemitism , warrior popes, support for capital punishment, corporal punishment, justifications of slavery, world-wide colonialism in the name of conversion to Christianity, the systemic violence of women subjected to men, and systemic violence such as poverty, racism, and sexism (see Violence in Christian Theology  by J. Denny Weaver , Professor of Religion at Bluffton (Ohio) College, where he is Chair of the History-Religion Department )

A good place to start analysing the roots of Christian-on-Christian violence is with St Augustine, especially from his letters.  Prior to Constantine, the only records of Christians being punished by Christian authorities for their beliefs were a small number of fanatics who were actually being punished for committing acts of violence for their cause, in order to become "voluntary martyrs", as described by the historian G.E.M. de Ste. Croix (see chapter 4 of his book “Christian Persecution, Martyrdom, and Orthodoxy” which can be downloaded from here... )

After Constantine, Augustine and other Christians in authority began the process of persecuting every religion other than their own, but also the persecution Christian sects that contradicted the doctrine officially promoted by the State.  For example, Augustine insisted that the authorities had a "Divine right" to crush the Donatists.

Augustine interpreted the "parable of the tares" from the Gospel of Matthew to justify the use of force against heretics.  Augustine said that the parable meant that the "tares" (heretics) needed to be uprooted - so long as this can be done without damaging the "wheat".

The historian Zagorin explains that "Augustine elaborated his position in favour of coercion in religion in a number of letters. In a lengthy epistle to the Donatist Vincent, he argued for the utility of coercion in inducing fear that can bring those who are subject to it to the right way of thinking."  

One significant example of the Augustinian "Christian theory of persecution" is to be found in a letter to Boniface, a Roman governor in Africa, dated 417 AD in which he says "If, therefore, we wish either to declare or to recognize the truth, there is a persecution of unrighteousness, which the impious inflict upon the Church of Christ; and there is a righteous persecution, which the Church of Christ inflicts upon the impious."   (See letter 185 here which also express enthusiastic support for coercion.)

Augustine embraced the idea that coercion, including violence, is a valid method for dealing with heresy.  He was simply going along with the established practices of his fellow Christians, who were already using fines, beatings, imprisonment, torture, and execution against various heretical groups and Augustine had been impressed by the results that had been obtained from these methods.