Paedophilia/incest; the most abhorrent of crimes ever.
When I started my first job, a young girl we worked with used to tell us about how her father (who was separated from her mother) gave her £5 in the mornings to have sex with her. We couldn't believe it, in fact - and shame on all of us - we didn't believe it. I had never even heard of the word 'incest' or understood what it meant until I was a big lump in my late teens, and when I did, I was horrified at the very thought of it and always thought back to what this girl had told us. But Jesus, WTAF?! Her father, the very man she should've been able to look up to, for love and protection from all the evils of the world? I still remember that girl's name to this day and often wonder how she fared in life.
Anyone who's guilty of committing such a crime, I hope they rot in prison for a very long time. They've ruined the lives of their victims, they've ruined the lives of families, they deserve nothing.