One argument atheists use to try to justify morality within atheistic worldviews is that we are simply wired with certain desires. Why is it wrong to murder? Because most people don't want to be murdered.
For most people, this is a double standard. Half the things on your plate did not want to end up on your plate either. But there is a greater example that even applies to vegans. Modern technology allows us to create virtual realities, which share a lot in common with our own. Take, for example, the videogame "Subsistence".
Subsistence is a videogame, available on Steam, wherein the player finds themselves in a wild space enclosed by a metal fence (this is the world border to stop players going into unfinished areas). The player must survive by searching for food and drink, and building a base to live in. The world includes a number of creatures, including one of the most recently added animals: The wild boar.
Each animal in the game is programmed with its own unique behaviour. The pig in particular is rather defensive. Usually, if it sees the player, or if the player attacks it, it will run away. However, if cornered, or if the player hurts it enough, it will charge at the player and attempt to hit them before retreating to a safe distance. It will also run from predators (though I'm not sure if it fights them like it fights players). This pig is completely focused on its own survival, and yet, it is not morally wrong for the player to kill it. The pig has been designed for a purpose. Its purpose defines the morality surrounding the pig.
Just like the pig, we are geared towards our own survival. That drive alone is not sufficient to establish the value of our lives. Unlike the pig, however, we have not been designed as an obstacle in a videogame. Rather, we are designed to relate to our Creator. Our lives are not valuable because we want them to be, we want them to be because they are valuable. We have been made in the image of God, and are designed to both represent Him here on the earth, and also to relate to Him as His children, friends and servants. That is what gives our lives value.
It is also what defines other aspects of our morality. We are designed to obey God, and when we fail, this is legitimately called evil. This is when God treats us like the pig. Not only does He have the right to kill us, but He is also right to kill us. In fact, it is unjust for God to not deal with evil. Therefore, when sin occurs, it must be dealt with. A death must occur. But whose death it requires is up for negotiation.
On the one hand, the sinner can receive the full penalty for their own sin. This is just, but it is not what God wants. Therefore, God provided a second option. 2,000 years ago, God the Son became incarnate, being miraculously conceived in the virgin womb of Mary. He lived a human life, with human weaknesses, but without the one thing that separates man from God. Jesus never sinned. Yet, Jesus died. He was put on trial, and was executed by crucifixion. With His last breath, He announced "It is finished", and then committed His spirit to God. On the third day, Jesus rose again.
In order to receive eternal life, one need only repent of the sin that brought us death, confess Jesus as Lord, and believe He rose from the dead. Doing so results in reconciliation to God. Upon our earthly death, we go to be with Him forever.