Umar ibn al-Khattab was one of Muhammad's closest companions and Sunni Islam's second "Rightly-Guided" Caliph. He was an early convert to Islam, and Muhammad married one of his daughters.
After Muhammad's death, a dispute arose as to who should lead the Muslim community. Most Muslims favored Abu Bakr. However, Muhammad's daughter Fatima sided with her husband Ali.
Unfortunately for Fatima, Umar had learned from Muhammad that disagreements should be settled through violence. When Umar stormed the house where Ali and Fatima were staying, Umar broke Fatima's ribs and caused her to miscarry Muhammad's grandson. She remained sick for several weeks before dying, just months after her father's death.
In the following video, Shaikh al-Habeeb explains why Shia Muslims hate Umar ibn al-Khattab. (Note: al-Habeeb refers to Sunnis as the "Bakri" sect, because they follow Abu Bakr.)