Mortal and venial sins

Some Lutherans sneeze whenever they hear anything that sounds like Roman Catholicism. Here is the Roman definitions of "mortal" and "venial" sins:

1854 Sins are rightly evaluated according to their gravity. The distinction between mortal and venial sin, already evident in Scripture,129 became part of the tradition of the Church. It is corroborated by human experience.

1855 Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him.

Venial sin allows charity to subsist, even though it offends and wounds it.

Or, mortal sins destroys "charity", which is another word for grace, while venial sins are what we might call "ordinary, decent sins". This distinction does not seem at all to be wrong. It is even scriptural, in that some sins, e.g. causing a little one to sin, make it better if one was not even born. I do not believe I have distorted the Roman view in the slightest. can anyone tell me why a Lutheran would reject these ideas? It does not call into question anything in the Gospel--it is simply an elementary distinction between the one who dies while shooting innocent children in the back and the one who dies after saying "F--- you!" to the guy who cut him off just before he goes over the cliff. I was speaking with someone who tried to say that this distinction is not right, that it is "Roman" etc. No amount of persuasion could shake him from this conviction. It is my opinion that this is due solely to an aversion to things RC--things like confession, vestments, chanting, and even some useful concepts like mortal and venial sins. This is not what we Lutherans are about. We are about Christ and his gifts to mankind, about his giving himself to us in Baptism, Holy Communion, about our sharing this unmerited love he has with us with each other and with those who do not know him. We should not define ourselves as "Not the Roman Catholic Church" but as the stewards of the fullness of the grace of God given through the Church. One is a "negative" definition, the other is who we are, and what we should present ourselves as.