Ok here is my personal line by line breakdown on the "meaning" behind "disarm".

"Disarm you with a smile."

Taking the power away from the abuser. They want you to hurt, they want power over you. Smiling into the face of the abuser "disarms" them. It takes away thier power over you, which is of course FEAR

This next line is a little more complicated as it uses 1 single sentence to relate 2 seprate but related thoughts. Please bear with me.
the first - "Cut you like you want me to cut that little child..."
(his strike back at the abuser,the first break in the repeating the cycle)

"that little child... inside of me thats such a part of you"
(the abuse of your own flesh and blood, how could you!)the pause after the word child is an artistic pause to create a dual meaning where the end of one thought is also the beginning of the next

"The years burn"
(Time goes by but pain dont stop. Burning Emotions of pain from within like a pressure cooker)

"I used to be a little boy , so OLD in my shoes"
(the abuse forced him to grow up long before he ever should have.)

"leave you like they left me here, to wither in denile"

(no one did anything, he was all alone, dying from the lies woven by the abused to protect the abuser)
"bitterness of one who left alone...ohh the years burn"
(the bitterness created by feeling you have allowed this to happen, self imposed isolation. Many people feel they "let" abuse happen, even when they have no control of it. Its how the mind "accepts" whats happening)these thoughts constantly burn inside scaring deeply)
"what i choose is my voice"
(breaking the silence)
"whats a boy supposed to do"
(stay silent or not)

"the killer in me is the killer in you"
(the abuser and the abused are tainted by the abuse. It was passed on. the abused becoming the abuser...)
"my love"
(The love for the abuser (stockholms syndrome))
'send this smile over to you x4"
(breaking the cycle, "disarming it" as he did the abuser)

Anyhow, thats what it says to me, Thats the great thing about music. Its a customized message to every ear.