It's hard to lose anyone you love, even when it is a dear pet.
I hope you show your pet mercy and allow him to end his life in peace. It will be hard, but show him the same love at the end of his life that he showed you throughout his life.
"When we go into battle, I will be the first to set foot on the field, and I will be the last to step off, and I will leave no one behind. Dead, or alive, we will all come home together." LTG Hal Moore, Jr., USA (Ret.)