In plain words
That is why Aṣṭamī, Kṛṣṇa's birthday, is regarded as a day that brings hardships. Elders say that those born on Aṣṭamī under the star Rohiṇī have to go through hardships. But there is also the view that such people can uplift the country. Kṛṣṇa punished the wicked, protected the good and established Dharma on earth. So Kṛṣṇa's birthday is indeed a most excellent day! It is a great holy day, not an impure day.
The killing of Śiśupāla
When Dharmarāja performed the Rājasūya sacrifice, he honoured Kṛṣṇa in the full assembly by offering Him the first honour of betel. Seeing this, Śiśupāla could not bear it. At once he stood up: “Dharmarāja! Many elders are seated in this great assembly. To pass over all of them and give the first honour of betel to this cowherd is not right”: so he spoke, censuring very harshly.
“The saris of those young women as they bathed: is it for carrying them off that You are worshipped? The day You splashed about with the flower-like cowherd girls: is it for the pleasure You took that arghya is offered You? Enough now; pack away this almanac of Yours; do not praise Yourself; stop; shut Your mouth!”
So saying, he abused Kṛṣṇa with whatever came to his tongue. Kṛṣṇa became angry. He hurled at Śiśupāla the plate on which Dharmarāja had set the first honour of betel and handed it to Him. It flew and struck off Śiśupāla's head. Many people think that Kṛṣṇa killed Śiśupāla by hurling His Sudarśana discus. But He did not use the discus. Whatever Kṛṣṇa takes up is the discus itself! He took up the plate and threw it. That very plate became the discus and cut off Śiśupāla's head.