But it begs the question of why ruin his combat prowess by getting his ass kicked so bad.
You give it more weight than it deserves, like a lot of people do.
I don't, because is obvious that everybody would've been badly kicked in the ass by an enraged Xebec.
An enraged Xebec who is protecting his family and being forced to kill his fellow Davys, to be more precise.
Shiki, Rayleigh, Gaban, Kaido, Linlin... all of them would've beaten to a pulp.
Garp, Roger & Newgate are the only ones who
could have done a bit better.
Why not have Xebec kill whoever was the Supreme Commander then instead?
I'm not sure I understand your point here