I am troubled by any unnecessary waste of life, particularly when acts of cruelty are committed against sentient species.
To be fair, the dolphins are not being treated worse than humans treat one another, so at least humankind is consistent. To be even more fair, dolphins treat other species (and occasionally one another) in equally cruel fashions. Occasional cruelty seems to be part and parcel with sentience.
None of which makes this right, and I do hope an end can be put to it.