1. Sentient is not a binary thing, but a more fuzzy ones. The sentience of apes or newborn for example is hard to quantify in a binary way.

2. Many magical creatures have a higher level of sentience than mere animals. Some are fully sentient like centaurs or acromentulas, some are half sentient like phoenix. Even magical owls or cats tend to be more sentient than their mundane counter-parts.

So it really seems from 1. and 2. that the level of sentience of unicorns has to be carefully evaluated, to be able to figure out if the harm done to them would be worth a "temporary cursed" life, it depends of the values of the three parameters : how sentient they are, how "temporary" it is and how "cursed" it is.