Basically, here's how to think about it:
A CBN ability cannot be undone or prevented at all. Now, since your Ruthless was CBN, no matter what your opponent did, its ability could not be undone or prevented. However,
their ability can't be undone by an evil card, which Ruthless is. So while your negate can't be undone, it can't undo something that is CBN, since that card can't be undone.
Does that make sense?
EDIT: Pol got here first...was trying to find a good non-Redemption comparison, but nothing was coming to mind