Just tying to figure out what your frame of reference is.

Whitechapel uses a lot of dark themes in thier music.

My interpretation: This song is centre's around the rage felt by those who were oppresed by a powerful figure. The knowledge that someone who caused suffering on a wide scale ,*dying in pain, might be the only justice some people get can cause normally good people to celebrateone's death. While others may feel the need to express themselves in a more corporeal manner. Some may feel they were cheated due to the offending parties not being tried and punished.

I don't think the song is saying you shouldn't forgive, and I do think there is a place to express negative views. I do believe they were trying to provoke thought on the subject of whether forgiveness after death is universal.

Edit: *