1. Go to this link for complete details http://en.wikipedia.org/wiki/Shambuka
The oldest mention of Shambuka occurs in the Ramayana of Valmiki, in the last book of the epic Uttara-kanda. After Lord Rama returns to Ayodhya and is crowned the king of Ayodhya, the death of a child occurs in the kingdom[1]. He is told that calamities such as this occur when Dharma is not followed in a kingdom. Rama tries to find out the reason and comes to know that a person of the Shudra jati, called Shambuka is performing penance which he is not supposed to do according to the Varna system of rules prevailing in that period[2]. He is executed, by beheading, personally by Rama[3]
The place where Shambuka was beheaded is identified as the hill of Ramtek, near Nagpur in Maharashtra.[4]
2. Killing of Vali "The Vanara King" from behind, like a coward
3. Doubting his wife Sita even after she underwent the Agni Pariksha
