What is the ruling regarding someone who feels compassion for animals that Allah has made permissible to slaughter, does not eat them, and does not approve of their slaughter, despite not prohibiting them?

Showing mercy to animals when slaughtering them is a beautiful, good character trait, so long as it does not lead to prohibiting what Allah has made lawful. Imam Ahmad and Al-Hakim narrated that the Prophet, peace and blessings be upon him, said: "And if you show mercy to the sheep, Allah will show mercy to you." As for refraining from eating them while believing them to be lawful, there is no sin in that; for a Muslim has the choice to eat what is permissible or to leave it. What is prohibited is to declare unlawful what Allah has made lawful, or to accuse the Shari'ah of oppressing animals.