Why should we not hurt or harm anything?
Hinduism teaches that God (or spirit) shines in the eyes of every living thing including our own, so when we hurt anyone we are actually hurting ourselves. This is why it is very foolish to needlessly hurt other living beings. Instead of hurting other living things, we should show respect for all living things, because it is the same God (or spirit) that lives in all of us.
The story of Ganesh and the cat
Ganesh is the son of the Mother Goddess. Like all children, he was sent to school to become clever. On the first day, he learnt the first three letters of the alphabet: A, B, C. He was very excited when he came home. He ran to his mummy saying, 'Mummy, mummy play with me.' But mummy was busy, She asked him to play by himself in the garden.
Ganesh went into the garden, and there he saw the neighbour's cat. Ganesh said, 'Cat, play with me. We are going to play the game of School. I am going to be the teacher, and you will be my student. I will teach you the alphabet.' In wobbly writing he wrote on the board A B C. Then he pointed at 'A' and said to the cat, 'Repeat after me, A'. The cat just went 'Meow'. Ganesh said, 'Cat: that is not very clever. Repeat after me: 'A, B...' The cat went 'Meow, Meow...' Ganesh was getting annoyed with the cat. He said, 'Cat! You must learn the alphabet properly, otherwise I will have to punish you! He then pointed at the letters on the board and said, 'Repeat after me: ABC. The cat went 'Meow, Meow, Meow.' Ganesh became angry. He picked up the cat and smacked it. The cat did not like this game anymore, and ran away.
Ganesh thought he would go back into his home. When he returned, he was shocked to see his mummy covered in bruises. Ganesh loved his mummy, he said, 'Mummy Who hurt you mummy?' Mummy said, 'Come here my boy, and tell me what were you doing just now?' Ganesh said, 'I was just playing in the garden with the cat. Mummy asked, 'Did you hurt the cat? Ganesh thought 'oh oh!.' He replied, 'Just a little'. The Mother Goddess said, 'Listen to me my boy, and listen carefully. This is the most important lesson you will learn in your life. I, the Mother Goddess, am the mother of the whole universe, and live in everything. It is I who live in the cat, so when you hurt the cat, you hurt me my boy. Do you now understand who has hurt me?' Ganesh now understood. It is God alone who lives in all living things, so if we hurt or harm anything, we hurt God. From that day on, Ganesh would never hurt any living thing.
Just like Ganesh, we must also remember not to hurt or harm anyone or anything. God lives in everything and in everyone. We must take great care of everything, including the earth, the plants, the animals, and especially all human beings.
Reference: Primary Hinduism by Seeta Lakhani (Ch.4, 'What Hindus Believe', p.49)