People who think that "life" - defined to include life in incarceration or similar states - is sacred for its own sake have completely lost sight of what's good about life. These people don't value what's actually good about life so they have to make up an artificial special case for it to make their moral system align with their pacifist judgements.
Life is a two-way causal link between a person and the universe, such that the person can affect their environment and be affected by it. Life is only valuable because it's necessary for the things we actually value, both moral values and amoral happiness.
A person sentenced to life imprisonment is dead. They just aren't allowed to accept their death and leave. And they certainly aren't allowed to reform if they were actually a criminal and make restitution; no, they're forced to spend the rest of their "life" eating food procured by the taxation of the innocent. It's a disgusting way of destroying a soul while forcing them to stick around in their body.
There's nothing magically immoral about killing; it's not inhumane or anything. It's just the best way to deal with a criminal whose crime is serious and can't be made to perform restitution.