1) He broke Omars nose after they stormed the house and said he would have killed him if he didnt take an oath from the prophet not to fight against them. I suggest you to read this book
It will clear many misconceptions for you inshallah. Also read kitab sulaym hadith 4 Here is a small excerpt from the lengthy narration ““Hadhrat Ali (AS) rushed, held him by the neck and pushed him away and Umar fell down and hurt his neck and nose. Hadhrat Ali (AS) intended to kill him. He remembered what the Holy Prophet (SAW) had said, and he said: “By Him, who gave Muhammad the status of Prophethood, O son of Sahak, if the Book from Allah had not been revealed and if the Holy Prophet (SAW) had not taken a promise from me before, then you would have known that you could have never entered my house.”
2) His short time in power was plagued by civil war after civil war. He was not able to do a lot of things because certain people would not stop trying to topple him.