Al-Hasan (may Allah have mercy on him) said: The reason the child born of fornication was named the worst of the three is that his mother said to him: You are not of the father to whom you are attributed. So he killed his mother, and so he was named the worst of the three.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19997