In my opinion, no dwarf becomes a banshee while faithfully adhering it his focus. If the dwarf is in a multi-generational project and then dies, then the focus is fulfilled. If the dwarf abandons the project, he might become a banshee.
A dwarf only ever has a focus that is within his capabilities. A dwarf enslaved at a young age makes a good slave because he ends up with a focus that relates to the circumstances of his slavery. Usually a dwarf slave’s focus revolves around serving his master in some capacity.