Why did Umar ibn al-Khattab say: "Kill Sa'd, may Allah kill Sa'd ibn Ubadah"?

Umar, may Allah be pleased with him, when asked: "Did you kill Sa'd ibn 'Ubada?!" replied: "Allah killed him." Umar's supplication against Sa'd was an idiom for turning away and neglecting, not actual killing. The motive for this was Umar's fear of the Muslims becoming disunited and divided, and their contravention of the hadith: "Thema'ims are from Quraysh," especially since the Ansar had gathered on their own in the Saqifah.