Não foi a Força. Foi pura vontade.
Vader convocou sua própria vontade para derrotar o imperador, como descrito no oficial Return of the Jedi novelização por James Kahn e > George Lucas Aqui está a passagem relevante da cena climática:
At that instant, Vader sprang up and grabbed the Emperor from behind, pinning Palpatine's upper arms to his torso. Weaker than he'd ever been, Vader had lain still these last few minutes, focusing every fiber of his being on this one, concentrated act — the only action possible; his last, if he failed. Ignoring pain, ignoring his shame and his weakness, ignoring the bone-crushing noise in his head, he focused solely and sightlessly on his will — his will to defeat the evil embodied in the Emperor....
He held the wailing despot high above his head, and with a final spasm of strength, hurled him into the abyss.
( Fonte )
Portanto, como você diz, este foi de fato um ato heróico de um único indivíduo se unindo contra a escuridão.