The world "Outlier" means a detached part from a whole. Something different from others.Thanks @johnjgeddes. I refer to those lines because that's where I got to understand the poem.
Romanians refers to venial as a small, considerable sins that can be make up for. But dark outlier to my understanding from this poem to me, is like a behavior that can be tagged evil, a heavy grave sin, such that can't be easily forgiven.
Meanings are hidden in those words. "Venial" "Expiated" "Outlier" "banished".