In one of the initial promotional videos, you could find that Bhallaladeva sitting on the thrown, while Amarendra Bahubali(saying by the minister sitting on the front row) is swinging the sword. In another frame you would find the younger Bhallaladeva rising from the thrown and shouting out Bahuali. In both the frames Shivagami is not visible. So Bhallaladeva might have somehow got the throne. But still it does not justify Kattapa killing Bahubali, since he was ready to rescue Devasena, by going against kings order in the first half.
1. Bahubali and Devasena conceive a baby
2. Devasena is captured by Bhallaladeva
Might be like in Mahabaratha, Kumsa tries to kill newly born lord Krishna but is rescued from the jail and also is crossed from the river, similarly Shivagami rescuses newly born Shivudu from jail where Devasena is captured. Might be some how Shivagami is involved in jailing Devasena and Bahubali tries to get her out by starting a rebellion and Kattapa has to kill Bahuabli to rescue Bhallaladeva(existing king). Might be the scar on his(Bhallaladeva's) face is also in the fight between them.
But its bad predicting the story, was expecting Kattapa to say Bhallaladeva's name in the end when asked who stabed him, since in the first half Bhallaladeva tells Devasena that there are only two people in this kingdom who remember Babhuabli's name, "you and me", me so that I could kill him once again. Might be he was talking about Shivudu or after stabing by Kattapa, Bhallaladeva kills bahubali.
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