The exact nature of Gotrek's crime has never been explicitly revealed; like many Slayers, Gotrek considers his transgressions an intensely personal shame, and has threatened to kill those nosy enough to pry.
In contrast to Gotrek's taciturn and absolutist mentality, Felix is a much more romantic, pragmatic figure, and frequently serves as the voice of logic and moral reason of the duo in order to remind Gotrek of the long-term consequences of his actions and guide him towards greater heroism.