I was originally raised in a conservative Lutheran church and household (Missouri synod split off church and later Wels). Your speculations on Justification is a big part of the picture, but there are other elements as well especially the other "Solas". Besides having major issues with Catholic Soteriology, Luther had major issues with the Magisterium, the pope as "Christ's representative on Earth" etc. Luther very much believed that the Pope was leading people away from the truth of the Gospel.
Having given my testimony I proceed to take up the bull. Peter said that you should give a reason for the faith that is in you, but this bull condemns me from its own word without any proof from Scripture, whereas I back up all my assertions from the Bible. I ask thee, ignorant Antichrist, dost thou think that with thy naked words thou canst prevail against the armor of Scripture? Hast thou learned this from Cologne and Louvain? If this is all it takes, just to say, “I dissent, I deny," what foo1, what ass, what mole, what log could not condemn? Does not thy meretricious brow blush that with thine inane smoke thou withstandest the lightning of the divine Word? Why do we not believe the Turks? Why do we not admit the Jews? Why do we not honor the heretic if damning is all that it takes? But Luther, who is used to bellum, is not afraid of bullam . I can distinguish between inane paper and the omnipotent Word of God.
"This teaching [of the supremacy of the pope] shows forcefully that the Pope is the very Antichrist, who has exalted himself above, and opposed himself against Christ, because he will not permit Christians to be saved without his power, which, nevertheless, is nothing, and is neither ordained nor commanded by God”.
Smalcald Articles, II
"nothing else than the kingdom of Babylon and of very Antichrist. For who is the man of sin and the son of perdition, but he who by his teaching and his ordinances increases the sin and perdition of souls in the church; while he yet sits in the church as if he were God? All these conditions have now for many ages been fulfilled by the papal tyranny."
Martin Luther, First Principles, pp. 196–197