It's Only (feat. Zyra) - Lyric Analysis

I heard the news today;
That you're not mine to keep.
Don't struggle too much now
While I kill you in your sleep.

She's found out he's cheating on her
Portrayed anger as a result

What is lust if it's not being by yourself?
I won't be gentle to the body on the shelf

Lust = unreciprocated love.
She feels discarded, put on a shelf and now hates herself for that.
The body on the shelf is her memory of him. The idea that if love is nothing then murder is nothing too. Her heartbreak was so awful it was equal to death and telling him to be careful next time as he is playing with fire when he breaks hearts.

It's only water
It's only fire
It's only love

Water and fire are opposites. They both can be used to destroy each other. In this instance she has now fallen in love with something that destroys her and her defensive response is to destroy it instead.

It's only slaughter
We're only liars
It's only blood

Continuously say and do things to hurt one another though it betrays their true feelings.

They're only thoughts that I'm having;
Thoughts safe within my head.
You're only crying
You're only dying
You're only red.

All the things that she wants to say, she knows it won't matter so she keeps it 'safe' in her head and therefore keeping herself safe. She's saying she thinks and hopes that maybe he'll feel bad and regret it. Their relationship is dead and his realisation will be too late.

I heard the news today,
That you weren't mine to save
I hope that you're comfortable
In the quiet lasting grave

Realisation she's now lost him. He's dead to her now.

What is lust if it's not being by yourself?
I won't be gentle to the body on the shelf.

It's only water
It's only fire
it's only love

It's only slaughter
We're only liars
it's only blood

They're thoughts that i'm having;
Thoughts safe within my head.
You're only crying
You're only dying
You're only dead

Thoughts which hurt too much she does't want to talk about them. She doesn't want to expose them in a way which will hurt him, but also does't want to completely keep them in either.

It's only water
It's only fire
It's only love

This is a hierarchy of destructive power. Fire is more destructive than water, and love is more destructive than fire.

It's only slaughter
We're only liars
It's only blood

They're only thoughts that I'm having;
Thoughts safe within my head.
You're only crying
You're only dying
You're only dead.

She has figuratively killed him within her head and since has has gave her purpose in life she now feels dead without him.