I'm all for being a veggie, and am almost there myself, apart from the odd bit of wild pheasant or something. Something that has a happy life and a quick end. But I would kill a suffering animal in a heartbeat, and in fact have had old or physically suffering dogs put down , and have even killed a half alive squirrel recently.
Global warming isn't the fictional bogey man many once thought it to be and its effects go way beyond marooning polar bears on melting icebergs. The methane (noisily) produced by livestock is 20 times more deleterious as a greenhouse gas than C02, which makes meat production the second most damaging source of greenhouse gases in the world.