Just because Luffy and co are the protagonists doesn’t mean all their allies are good people, most are downright insane.
The grand fleet is made up of:
1. Aman who revels in murderer - Bartolomeo
2. A mass murderer who didn’t care about killing people just happy to gain fame - Cavendish
3. A literal mafia group
4. A war Criminal - Suleiman
5. Bounty hunters known for brutality
And more.
Yeah thats true, even the SHs themselves were pretty gray area, especially the likes of Robin and Franky ofc. Well Jinbei included now too.
I was shocked Oda had those guys join the Grand Fleet but as you said, they are insane and thus fit into the lunacy of the SHs especially Luffy the craziest one himself lmfao.
I know Barto did some heinous shit but I didnt know it was that bad, when you mentioned about him burning people alive, holy shit. Although we saw him trashing that town in the cover page, the Shanks territory one People think hes a cannibal too because of that nickname but AFAIK, that is because the word used for it, means "troll" in Japanese culture, as in the online slang word? So hes not a literal cannibal? But then again after Big Moms backstory, Oda is definitely not afraid to cover that or Zeffs backstory with Sanji to think of it, yikes lol.
I should have been a lot less suprised they would team up with Capone then at WCI to think of it.