Saw death on a sunny snow)
This song is set up like a play; with this line he is setting the scene. So apropos basically means so, by the way. This line is saying he experience something painful on a snowy day.
In this situation he could be using apropos to mean Very appropriate to a particular situation. As he is saying it is very fitting to see death on a sunny snow something that is bright, white, and pure, one can interpret this statement as being contradictive, perhaps said semi-sarcastically.
To help improve the meaning of these lyrics, visit "For Emma" by Bon Iver Lyrics and leave a comment on the lyrics box