There is no need to "justify" killing animals for meat.
There's a little something known as the food chain; are you going to condemn a wolf for killing a deer, or a lion for pouncing on a zebra? If you choose to be vegetarian, then fine; that's your choice. However, you are still causing the deaths of animals, even if you grow your own vegetables. If you put down slug pellets, or use any kind of pesticide, you are killing animals. There's no point in getting on a high horse about this.
Also, how do you know that plants don't feel pain? There's always that possibility.
Do Plants Feel Pain?