Just why does the series have a pumped-up, Hot-Blooded opening theme contrasting with a soft rock ending? The final episode drives the point home - Yuusuke, finally unburdened by the duty of being Kuuga, shows his kindness to a group of children with the ending theme playing over. The two musical themes of the show present the character of Yuusuke Godai - a kind man forced by circumstances to become a fearsome warrior who would save the world.
The fact that four of the Grongi are based off Kamen Rider, namely the Grasshopper Grongi, Gadoru, and Daguva, makes it practically foreshadowing for the fact that Kuuga's powers is no different from them.
The final Good Old Fisticuffs battle, when Daguva was gleefully laughing, suddenly you realized that N-Daguva-Zeba wasn't fighting to win, he was hoping that Yuusuke loses what's left of his humanity in that fight. Kuuga was also a Grongi himself (an immature one) and Yuusuke was simply the human host for the transformation. The final fight was to truly bring out Kuuga as a Grongi so he can usher in an eternity of violence.