Why did Kattappa kill Baahubali?

There are a few dialogues in the film, which could give an answer to this question. So lets just rewind to some scenes.

1. The first scene: Where Sivagami takes the child and falls into the river. She says,"for the sins i have done,let my death be the punishment ... Let this Child live, Mahendra Bahubali should live"...

This implies that she has done a sin, which could be the death of Amarendra Bahubali.

2. The Scene in which Bhallaala Deva talks with Devasena. He says, "We both are waiting for Bahubali to return, so that you can see him for one last time, and I can kill him once again with my hands".

This implies that, Bhallaala Deva himself killed Bahubali. (He is a warrior. He need not take credits for the killing, if Kattappa was the one who killed Bahubali after all). So, this could mean that Kattappa just lied and it was Bhallaala Deva who killed Bahubali.

3. Why should Kattappa lie?

Maybe because Sivagami Asked him to do so.

4. We see Bahubali being killed from behind, by Kattappa. Kattappa is a Warlord. He is brave. Why would he kill someone from behind? Maybe he mistook him for someone else? Maybe?!

This answer contains spoilers, so please don't read ahead if you didn't watch Baahubali: The Conclusion yet.

Reason: Shivagami orders Kattappa to kill him after she was told by Bhallaldeva that Amarendra wants to assassinate him.

In the end of the Baahubali: The Beginning, we saw that Amarendra Baahubali was made the king and Bhallaldeva as commander-in-chief.

As explained in the new movie, Bhallaldeva and Baahubali wanted to marry the same girl, Devasena. So, Bhallaldeva asks his mother Shivagami. Therefore, she sends the marriage proposal to Kuntala king who is brother of Devasena. However, Devasena rejects the proposal with harsh words, after which Shivagami orders to bring her as captive.

Shivagami gives Amarendra an ultimatum to choose either his position of king or Devasena. He chooses Devasena and becomes commander-in-chief.

Being a king, Bhallaldeva relieves Amarendra from his duties. After that, he creates a plot and tells his mother that Amarendra wants to assassinate him.

Thereafter, she orders Kattappa to kill him. Due to bound by words of duty for kingdom, Kattappa kills him under a false pretense.

Amarendra Bahubali and Bhalladeva both fall in love with same girl; Devsena. But Devsena chooses Amarendra Bahubali as she falls for his kind nature; loosing both the kingdom and girl made Bhalladeva frustated and hence, "Bhalladeva" with the support of his father, plots conspiracy against Amarendra Bahubali, and manages to convince the queen Sivagami and she orders Kattappa to kill "Amarendra Bahubali". Even though Kattappa likes him, he has to follow his queen's order, and so, he kills Amarendra Bahubali. He regrets this all his life and waiting for a chance to correct his mistake. 2. After queen orders Kattappa to kill "Amarendra Bahubali", she realises the true intention of Bhalladeva; only to be late enough that "Amarendra Bahubali" is killed and Bhalladeva trying to overtake the throne and kill the child too.

The movie Bahubali: The Beginning is the first movie of the series of three movies that has been planned to be released in 2015, 2016 and 2017.

So we could say that why Kattappa killed Bahubali is still a mystery which might be revealed in the coming two installments in the series, which is called "Bahubali: Conclusion"

Watching the first movie doesn't give any clue on why he did that, the first movie is the starting and there were no clues in what fight did Kattappa killed Bahubali after the war in the end. In the end Bahubali is shown to become the next king, but what happened between it and the fight in which he was killed was not shown, so it's impossible to tell.

Just wait for the next movie for the conclusion.