Are cows killing our planet

That all depends on your perspective. The national herd in both the United States and Canada is declining, but it is increasing in Australia, New Zealand and South America. India can easily be considered over-populated with cattle because its religion doesn't exactly promote the control of its native cattle population when there are numerous such animals living in the streets and in the countryside. With other countries, though, there are areas that could use more cattle than they currently have, though that's just based on opinion.