Magnificent Bastard: Grandfather wasn't always this, but his first defeat in the movie's prologue evidently taught him some lessons about being an Evil Overlord that he took to heart. To prevent the children of the world from rising against him again, he decides to turn them all into Senior Citi-Zombies, starting with the Kids Next Door; to make sure the adults of the world don't try to stop him either, he does the same to them — including the other villains of the show; even when he has more than 99% of the world under his thumb, he swears not to rest until he's destroyed the Book of KND, which was, after all, the thing that allowed Numbah Zero to defeat him before. Once he meets his rebellious son again, he declares he won't give him a choice between cooperating-or-else again and tries to bring him down.
Both his sons show signs of this themselves. Father brought all the adult villains of the world together and attacked the KND Museum and Moonbase simultaneously, knowing that Sector V would prioritise the Museum on Numbah 1's orders. He also knew ahead of time that the Recommissioning Module was not actually damaged beyond repair, and pre-empted that Numbah 1 would come to their Convention Centre to fetch it — and that he would come alone, without any back-up. Not only that, but he made sure that Numbah 1's "right on schedule" arrival would benefit his Evil Plan — he provided a KND operative's genetic material necessary to make the Module work. As for Numbuh Zero? He distracted Grandfather while Numbuh 1 and his team crashed the Moonbase right on top of him! It didn't kill him, but that wasn't what Numbuh Zero was trying to do — he simply wanted to get him into a Decommissioning Chamber, allowing them to De-Power him once again.
Nightmare Fuel: The senior citizombies' yellow pupiless eyes are quite frightening.
Each of the KND getting infected, but particularly Numbuh 362, considering that she's the strongest member of the KND.
Tear Jerker: Numbuh Zero purposefully broke the Recommissioning Module so that he could finish the most important mission in his life: being a good father to his son, Numbuh One.