You go to see a magnum opus. It starts to pull you in from the word go. And just when you think you have every valid reason to root for the protagonist, they give you possibly the ultimate reason to double up on that rooting: They kill off a character pivotal to the story, whose death our hero must now avenge, no matter what.
There is no doubt that the two Baahubali films constitute a magnum opus. And by the end of Baahubali – The Beginning we have enough to root for Mahendra Baahubali: His father was killed – possibly by treachery – and his mother is imprisoned under humiliating and inhuman conditions. So why are we talking of Kattappa's death?
Here's a disclaimer before you go any further: What you are about to read is a THEORY – but a pretty strong one in my opinion. I have not seen the film, but I am making an educated guess. This is based on years of watching films, some fewer years of reviewing them, and being a total fanboy when it comes to Baahubali. So, here goes...
Death as a motivation
Personal tragedy has been a great motivation for protagonists in films in India. In Hindi films in the 1970s and 80s, it used to be the rape and/or murder of the hero's sister. Filmmakers had this down to a trope after a certain point of time. As time progressed, this was turned into the death of a parent or friend. And it worked wonders: Remember how you supported DJ and his gang in Rang De Basanti after Ajay's death? In more recent times, films witnessed a two heroines trend, and the death of one spurs on the hero: Remember Ghajini?
Now, one may argue that such a death has already occurred in Baahubali: Baahubali Senior is dead. And that scene will play out once again on screen in Baahubali – The Conclusion. So why can this death be not a motivation? The answer is simple: It does not fit well with the three-act structure that all films follow.
Crucial second and third acts
Now, Baahubali – The Beginning had three clear acts. The first was the entire first half before interval, the second was the flashback where Amarendra Baahubali and Bhallaladev grew up, and the third was the fight with the Kalakeya.
For Baahubali – The Conclusion, the first act is definitely going to be the remainder of the flashback, where Amarendra Baahubali and Devasena meet and fall in love. It is not clear whether the civil war – it can be called little else because Sivagami herself terms it Antaryuddh in the trailer – takes place in the first or second acts.
Assuming that less time is spent in the flashback, the second and third acts can be expected to be the story of how Mahendra Baahubali avenges his father's death and fulfils his mother's wishes to see Bhallaladev humiliated and punished.
No other death suffices
The revenge saga that Baahubali is, its makers would not want to kill off characters like Devasena, who is yet to see her husband's death and her own humiliation avenged. Mahendra Baahubali will also not die: It makes much more sense for the makers to keep him alive. No point in keeping Avantika alive either: Her character is not fleshed-out enough for the audience to commisserate with her and root even more for Baahubali. Also, all three of them need to be alive by the end of the film to be the complete and satifying revenge saga Baahubali is shaping up to be.
The only major protagonist left, then, is Kattappa. He is universally loved, despite revealing that he is the one who killed Amarendra Baahubali. He is also quite fleshed out as a character: People are already rooting for him. Thus, if he ends up being killed by the end of the second act of the film, the audience would be outraged enough to bay for the blood of Bhallaladev and his men, and for Baahubali to do the honours.
Kattappa's death could also signal the end of the servitude he and his family was bound by when it comes to the Mahishmathi royal family. And that end will come via a pardon by none other than Baahubali himself!
So there you have it. Kattappa will most possibly die by the end of the second act: midway through the second half of the film. Again, this is an educated guess. Let's see if this comes true!