why did Antony and Octavius kill the senators?

Octavius and Antony composed a list of 70 senators that would die. These senators posed, they felt, a threat to their power. Cicero's name was among them. After the death of Julius Caesar, Cicero began speaking out against Mark Antony, so Antony decided to have Cicero and his supporters killed. Cicero was 63 years old when he died.

Octavian and Antony agreed to share power. They, along with Lepidus who was the Pontifex Maximus, were designated by the Senate as a Triumvirate with dictatorial powers for five years. Octavian, Antony and Lepidus agreed to eliminate the Senators and members of the Roman aristocracy whom any one of the three considered a threat to public order. 300 senators and two thousand lower level aristocrats were executed by their orders.

source: http://www.sjsu.edu/faculty/watkins/caesaraugustus.htm