If Oda wants to make Blackbeard looks actually evil, he can just reattack Amazon Lily now. Just like when Whitebeard sent tsunami to Marineford.
Seeing how prime Whitebeard sneezing actually caused an earthquake, Blackbeard can attack the island from his ship, and Rayleigh, Hancock, Shakky, etc won't be able to do shit from the island.
Gura Gura is hyped as "able to destroy the world" and "strongest paramecia" for reasons.
After the quake and tsunami attack, Rayleigh and Hancock will at least get injured seriously, then Blackbeard comes to send the finishing blow and get Mero Mero no Mi.