Yeah, I agree. I didn't read AvX, but I did read Hickman's Avengers (though it's been a while) and it bothered the hell out of me that these two ended up on, essentially, friendly terms. Especially with Hickman doing his own little Crisis event, he could have had T'Challa kill Namor then had it undone at the end. I don't care what line Marvel tried to sell us, Time Runs Out and Doom's little patchwork planet counts as a continuity reset and Namor could've been alive again when the dust settled.
I know Namor is squared into that anti-hero realm so the bad crap he does requires an out, but *even if* the Phoenix was influencing him and *even if* he was just talking sh*t because he'd rather have T'Challa think of him as a monster rather than a victim, T'Challa was well within his rights to kill Namor, and I think it'd have been in character for him to do so.
Granted, killing Namor isn't exactly an easy feat, but it's not like that's beyond the Panther.