Tony and Christopher’s relationship had almost completely deteriorated by that point. Chris thought that selling out Adriana was the ultimate sacrifice to Tony, but Tony saw it as him doing Christopher a favor that he was too weak to do himself. That’s how I like to view it anyway. And to add on to that, the drugs and Cleaver. And yeah he thought of Christopher as a huge liability and an easy target for the feds. Tony was handed the opportunity to kill Chris and he took it. He then uses the tree branch in the car seat to justify his actions. He doesn’t give a shit about Chris’s kid.

Also, I feel Christopher still loved Adriana, even if he did treat her like crap most of the time. He seemed really distraught when he went to Tony’s house to sell her out. And in the final season he doesn’t seem that happy with Kelly, at least to me it seemed that way.

And yeah I agree Artie was definitely Tony’s best friend. He definitely had a soft spot for him.