Would renal failure result in hypo- or hyperkalemia? Why?

Depends. Many factors contribute to serum potassium levels including residual kidney function, diuretic use, potassium intake, GI potassium loss (i.e. diarrhea), and acid/base status. It is possible to see high or low serum potassium levels in patients with renal failure but hyperkalemia (high potassium) is more commonly observed in patients with advanced renal dysfunction.