Yep, the man Ellis has started to care far more than a friend is gone and it wrecks the man apart that he's going to put the zombie down. It's either let him bite and join him in zombie servitude or...

"Sorry sugar, but I ain't lettin' ya go."

And that, he grabbed the nearest jagged rock and threw it squarely to Nick's forehead. This was to make him stumble upon his feet, and crack the skull before he shoved him onto the spike. He had to make it clean, but no risks of lounging.