To kill cows means to end human civilization.

Srimad Bhagavatam canto one chapter four verse nine 

abhimanyu-sutam suta

prahur bhagavatottamam

tasya janma mahascaryam

karmani ca grnihi nah

abhimanyu-sutam--the son of Abhimanyu; suta--O Suta; prahuh--is said to be; bhagavata-uttamam--the first-class devotee of the Lord; tasya--his; janma--birth; maha-ascaryam--very wonderful; karmani--activities; ca--and; grnihi--please speak to; nah--us.

                              TRANSLATION

It is said that Maharaja Pariksit is a great first-class devotee of the Lord and that his birth and activities are all wonderful. Please tell us about him.

                              PURPORT

The birth of Maharaja Pariksit is wonderful because in the womb of his mother he was protected by the Personality of Godhead Sri Krsna. His activities are also wonderful because he chastised Kali, who was attempting to kill a cow. To kill cows means to end human civilization. He wanted to protect the cow from being killed by the great representative of sin. His death is also wonderful because he got previous notice of his death, which is wonderful for any mortal being, and thus he prepared himself for passing away by sitting down on the bank of the Ganges and hearing the transcendental activities of the Lord. During all the days he heard Bhagavatam, he did not take food or drink, nor did he sleep a moment. So everything about him is wonderful, and his activities are worth hearing attentively. Desire is expressed herein to hear about him in detail.