Why Katappa killed Bahuballi?

For those who can't wait for it badly:

Background:

Bahubali owns mashismati kingdom, bhallaldeva and bijjalladeva jealous about the victory.

So they actually planned both to kill rajmata in a dramatic way and Veer bahadur kumar verma (which is very close to bahubali and his wife Devsena) has listen this gossip and  bijjalladeva slapped his father to tell don't dig into this matter and emotionally character played by bijjalladeva in front of Veer Bahadur to provoke to kill his own son (Bhallaldeva) so that they will tell Rajmata that veer bahadur came to kill Bhallaldeva from Bahubali order. 
So below is the actual story:

Katappa: Is this happening with your consent? I beg you queen mother. Ask the king to take back his command. Even integrity can take a backseat. Bahubali has no mean bone in his body. The son you raised. He grew up drinking your milk. The Dharma you taught him runs in his blood.

Rajmaata: Bahubali has to die.

Katappa: No, I can’t do it. He gave sword to rajmata as punishment for my refusal. Please sever my head

Rajmata: Will you kill him? or shall I finish this task?

Katappa: No queen mother. Your hands should not be stained by that sin. I will kill him.

Bijjaladeva: Can we trust this Dog bhalla?

Katappa: and he killed bahubali 

More you will need to watch the film the actual scene :)

Cheers!

Enjoy