Christ did not appoint any bishops or had any intention in doing so and using it as power tool to control nations. He despised power and hierarchy and was very libertarian in that respect.
When Constantine saw Christianity was gaining ground in the empire, they`ve adopted it as official religion to be able to control their empire better. After viciously persecuting Christians, they`ve decide "if you can`t beat them, join them".
St. Constantine didn`t give a rat`s ass about Christians or Christianity. For him religion was just a power tool. Christianity became a real threat at the time for several reasons, so he decided it`s best to crown himself high priest and control it rather than to fight a losing battle.
Saying he deserved to be a saint, being son and wife murderer is ...emm hard to put into words. He would have gotten the electric chair had he lived in today`s Texas.
But Christ did appoint twelve, right? And then He appointed Apostles, right? And these Apostles, following His commands, appointed elders (bishops) and presbyters and deacons, right? And they did this by the laying of hands, correct? Why do you ignore these facts?
And when St. Constantine became Emperor, Christianity was AT BEST about ten percent of the population, and a population which was at that time considered hated enemies of the state and persecuted by the vast majority. Seems to me he took a great risk in proclaiming equal rights to Chrisitans and proclaiming himself a believer in Christ, but you wouldn't know that because your blind bias shields you from the truth.
And no, St. Constantine never considered himself as some high priest like you say. In fact, he respected the authority of the bishops as divinely ordained overseers of the Church and at the First Eucemnical Council, which he called in order to stop the great division being caused by the Arain heresy within the Church, he went as an observer and had no vote.
So you see, if you spent a little more time learning history as it is instead of accusing and judging people, especially ones who you in no way have the foggiest understanding of what world they lived in during those times, then maybe you wouldn't make such inaccurate statements.
King David murderd as well, as he is one of the greatest saints of the Old Testament because he repented. Who are you to judge someone like St. Constantine (who it is not even agreed upon by scholars that he was indeed involved in the murders you listed above) and what the condition of his heart and soul was in his waning hours of life. Indeed, the repentant thief on the cross entered the Kingdom and St. Constantine did more to save Christians then the thief ever did.