Vegetarians think that killing animal for our own comfort is cruel because we can live even without eating meat then why to kill animals. They are also living creatures as we and killing them is equally painful for them as for us. I need answer in context when someone does not believe in Quran/Prophet etc.
Then "killing" plants also does not seem right to me as they're living things too like us (keeping aside if they feel pain or not). We wouldn't have survived till now if we were not to eat them killing mercilessly.
Things are perfectly defined as what are good and what are bad in Islam.
We check the condition of the animals before slaughtering (healthy, fit, young etc.) because we're told to do so.
Don't you think deceiving a cow while getting milk is wrong? Might be, but we're not obliged to not deceive it, so we have it. Don't you think killing plants is wrong? We're not obliged to not kill them, so we have them.
We're not obliged to not kill animals, so we have them.
O ye people! Eat of what is on earth, Lawful and good; and do not follow the footsteps of the evil one, for he is to you an avowed enemy. (Qur'an 2:168)
Besides, having meat is not a compulsion (AFAIK). Just because they're good for our health (and tasty). Causing them pain intentionally should not be our intention.
Allah knows the best.