While Martin Luther made some doctrinal errors, such as his rejection of certain sacraficial aspects of the Mass, Luther did in fact rightly condemn (along with Catholic humanists such as Erasmus) the abuses of the time. Although he fell into error, it should be noted that the Popes of the times oftentimes whored around and bathed themselves in the blood of fellow Christians. To Luther and many of the Reformers, Rome itself had become pagan (just witness the “pagan” art pieces then being commissioned in the Vatican and other Italian cities).
It should also be noted that much of what Martin Luther argued for was later adopted by the Church at Vatican II: vernacular language for the liturgy, communion under both kinds, a de-emphasis of language on the Mass as sacrafice, more Christo-centric, less juridical approach to the Church, etc.
I do not think it was the intention of Luther to divide the Church. Luther was excommunicated; he did not leave the Church. Given the abuses of indulgences, the nepotism, un-Christian living of the Popes, revival of interest in things pagan, etc. then prevalent in Europe, the Reformers wrote against this but also came to the conclusion that the Popes and bishops were simply human men who could make mistakes and even err in doctrine.
Why, if the Church were One and undivided, the Reformers might have asked, are the Greek Orthodox not part of the Church? They have the same sacraments, the same faith, but are believed to be damned because they are not in communion with this one man, this Peter, who is clearly shown not to be without error, much less the Vicar of Christ, the one God chose to rule over the Church. The visible Church already is shown to be divided, and it cannot be identified with any one visible structure, for there are the Roman Catholics, the Greek Orthodox, even the Oriental Orthodox–and all these profess to be the One Holy Apostolic Church and yet are divided.