Remember when Tone gave that bit of foreshadowing during Christopher's intervention? 

"You killed little Cosette, I should suffocate you, you little prick!" 

A few seasons later, you see the camera pan to the empty baby seat in the wreckage of Chris' car before Tone decides he is too much of a hopeless liability to be left alive. 

TLDR: Chris was the "son he never had," but T seized the opportunity to put him down for the sake of the child.

https://i.poastcdn.org/218aeea0c83deef231b0f35953cdeda38b42653d2c2759362ab12c688e5eb879.png

https://i.poastcdn.org/0aa27cbfee9a919a3935d15d4ee8d1e69720b0a844dfb2e6857572c7323f329f.png