One of the main differences between pets and people is that pets are considered property. Simply put, by most countries laws, they're no different then the computer you own and yes, you can do damn near anything that you want with them.
People though are different. Kill them and you've committed murder. It's something I recently had to seriously discuss with my family in regards to mom. She's currently in a nursing home due to a broken ankle and will be there for several weeks before it heals. One of the questions they asked is what level of Resusitation would we allow and that took some serious discussion before we decided that at that point, she's gone and authorized "DNR" or do not resusitate. This means if she has a heart attack, stroke or other issue that ends her life "Then So Be It" but we will not allow her to demand or them to suggest being put down in a medically assisted death.