The tank will not cycle just by letting it sit empty a few days. You need a food source (fish waste for a fish-in cycle, or pure ammonia if you do a fishless cycle) for the bacteria to feed on and grow.
IMO there is nothing cruel about using fish to cycle if you use a few and go slowly. 6 danios in there for a month would do it. Then add more fish slowly over time.
What other fish, apart from angels, did you have in mind?