Namor didn't have to be killed, because there is war at stake, a war with an equally vibranium-yoked up nation. There was more at stake than a personal beef. The Wakandas where looking like they were about to be wiped out before Shuri showed back up with Namor to end the shyt.
Your logic is all over the place.
Namor's fighters were more capable from top to bottom. Even after Okoye "killed" some of them on the bridge, they got right back up like nothing happened. So your solution is to start a war with these people that get right back up after being "killed", after you've take consecutive L's to them, when Shuri chose a better "play nice" option to stop the Ls.
"but Namor had to be killed"