Eusebius was kind enough to write a history on him See here. I figure it will be much more accurate than any biography written these days, especially considering he wrote it only a hundred years after he lived.
I’ll tell you the formula used to smear Constantine as a bad influence on the Church.
#1- He was a pagan
#2- It is/was questioned that his conversion was a political ploy. Most of this thought comes from the fact that he was such a high profile and it did indeed have an enormous effect on the Roman empire. There is actually no evidence (that I have seen), other than a hunch, pointing to the fact that he was not genuinely converted.
#3- He was an emperor not a religious leader, and anyone seeing a secular leader having such an impact on religion as him assumes it was a bad impact.
Regardless of what he actually did, these three things add up in many peoples minds as enough to write him off as bad.
I personally like Constantine. I feel he really tried to lead an empire into Christianity.
Also it should be noted Eusebius was credited with a work called Oration in Praise of Constantine. So a major church leader (bishop of Caesarea) has good things to say about him less than 100 years later. Pretty good indicator of whether the impact he had was good or bad.