From the first season, The Doom Dimension! It is scary looking and in Dan and Masquerade's second to last battle against each other after beating Dan and sending him and Drago to the Doom Dimension, Masquerade himself says that the Doom Dimension is the afterlife for Bakugan! So basically he's sending Bakugan to hell!
Coredegon/Wiseman/Mechtavius Destroyer. Just...DAMN. If you thought Mag Mel was Nightmare Fuel, Coredegon is worse. Besides having the highest body count in the series, Coredegon has pulled off several events that can leave you terrified.
During Countdown to Doomsday, Mechtavius Destroyer manages to defeat Dragonoid Destroyer for the first time. It's pretty unnerving to see Dragonoid Destroyer, who effectively curb-stomped Mechtavius Destroyer effortlessly TWICE, suddenly be overpowered and trashed by Mechtavius Destroyer.
To make matters worse, Mechtavius Destroyer tells the Brawlers to KILL THEIR OWN BAKUGAN PARTNERS (Naturally Dan, Marucho, and Shun don't do this)! What happens if they do? Mechtavius Destroyer will still kill them, but only them. What happens if they don't? Mechtavius Destroyer will kill the entire population of Earth. JESUS, Omnicidal Maniac indeed.
When Slycerak, Exostriker, and Mandibor are revived and battling Dan, Marucho, and Shun, Coredegon (in his Wiseman form) kills Balista for finding Gunz. Unlike the other Nonets he kills, however, WE DON'T ACTUALLY SEE COREDEGON KILL BALISTA.
During the battle between the Battle Brawlers and the Nonets, Wiseman expresses a Slasher Smile as he gets closer to ressurection, and his stoic demeanor has clearly vanished. This is only topped when Wiseman begins cacklingmadly as he transforms into Coredegon.
One Nonet's death worth noting is Mutabrid. When Mutabrid, Betadron, and Kodokor attempt to combine, Mechtavius Destroyer, in a showing of Genre Savvy, blasts them to stop them from doing so, and then proceeds to severely injure Mutabrid. As Mutabrid struggle to combine with his brothers, Mechtavius Destroyer proceeds to blast Mutabrid again, causing him to scream in agony. However, before Mutabrid can return to ball form, Mechtavius Destroyer BLASTS Mutabrid one last time, effectively killing Mutabrid. Fridge Horror kicks in when you realize Mechtavius Destroyer could have done this at anytime, but chose not to in order to give his enemies false hope.
After Gunz rejoins the Brawlers, Worton says that Mechtavius Destroyer is on the move. We then see Mechtavius Destroyer arrive at Bakugan City, where he unleashes an energy dome around him that engulfs Bakugan City. After the flash of light ends, we see Mechtavius Destroyer has completely annihilated Bakugan City, along with everyone in it. By the way, prior to this event, no human had ever died in Bakugan.
As the Brawlers go down the Current of Time to prevent Mechtavius Destroyer's slaughter fest from happening, we see that Mechtavius Destroyer eventually heads to Neathia, Gundalia, and Vestal since there are Bakugan there and lays waste to all of those areas. Keep in mind that he must have also killed Fabia, Rafe, Ren, Paige, Spectra, Gus, Ace, and Baron since they would've tried to resist him. Thank goodness Dan, Marucho, Shun, and Gunz stopped all of that (minus the Nonets' deaths) from happening.
Now, let's count how many named characters died (and would have likely died too) at Mechtavius Destroyer's hands before history was changed for the better. Fridge Horror is in effect here.