Complete Monster:Vulgyre serves as the true antagonist. A plant-like alien who once coveted the hand of Princess Meadow, after she refused him, he took her life. Later impersonating her, he would use her as his figurehead, creating the Galactic Imperial Army Zone with the purpose of destroying 1000 inhabited planets with the hope of becoming immortal and the God of the Milky Way. After succeeding in destroying 999, he made his way to Earth to complete his goal. He forcefully transformed his general Doldora and her subordinate into a mindless monster after they discovered the truth about him and refused to continue working for him. He helped turn Chevalier into a killer to corrupt his soul with the blood of his victims, with the purpose of killing him off later to gain a power-up. Later becoming Galactic Super Beast Vulgyre, he launched an attack on the city and it was shown that he had kept the soul of Meadow from moving on just so he could have her for himself. Vulgyre is obsessed with pleasing only himself, uncaring to the countless innocents whose lives that he took and has one of the highest body counts in the history of the franchise.
Hilarious in Hindsight: A team named Gingaman shows up in episode 9 (turns out they're really villains), 8 seasons before Gingaman was a name of a real team of Super Sentai.
Moral Event Horizon: One episode has Doldora using her Monster of the Week to create an illusion of the Fivemen's mother in order to set a trap. Seeing the way Remi holds onto the hope that her mother might be real is enough to send Doldora over.
Billion gets a sword that makes its user Brainwashed and Crazy, as well as giving them evil powers. He gets one of Gaku's kendo students to use it, preying on the fact that the kid wants to be stronger in order to protect his parents because he knows the Fivemen won't kill the kid.
Then we have Garoa, after destroying the Fivemen's Humongous Mecha, but not the Fivemen themselves, he decides to draw them out, by attacking their school full of children. The scene with the children running to escape like they're about to die cements it. Shortly before these happens, he has dropped his Butt-Monkey status, if you didn't notice.
The true villain of the season, Vulgyre, killed Meadow when she refused his offer to rule with him, then impersonated her for the entirety of the season while destroying planets just to become all-powerful.