I replayed the story mode recently and noticed something new. When the Dragonborn learns a shout he/she also learns the understanding of who taught them the shout. Before the Dragonborn learned the Dragonrend shout he/she was warned that by learning this shout, the Dragonborn would also accept the understanding of what this shout meant.
The Dragonrend shout was made by men out of complete hate and despise for dragons. And made during a time in which dragons tormented and massacred men. Once learning this shout the Dragonborn accepted this hate and inability to forgive Parth for all of the unspeakable atrocities that he committed to the men that created the understanding of this shout.
I believe that Arngeir warned the Dragonborn and didn't want him/her to become corrupted. People like to compare Parth with a high ranking Nazi general; imagine if this reformed Nazi general opened up an organization to help the families of concentration camp victims. It would be easy for us to think "huh.. maybe this person deserves a second chance to repent for his crimes", however what if you were a concentration camp victim during WW2 and held that understanding of deep rooted hate and despise; you would want that mothersensor to rot in hell.
Think about that for a little bit.