Yes,Umar was a cowered in the balltefield, somthing sunnis testifie to: 

  
 When the news of the Holy Prophet's death got wind, panic seized some of the Muslims, and thinking that  all was over, they fled to Madina According to tradition when Umar heard the news that the Holy Prophet  had died, he flung his arms declaring that it was no use fighting any longer.  

 It is related that Anas b Nadar passing by Umar asked him how it had fared with the Prophet. Umar  replied that he had heard that the Prophet had been killed. Anas observed that the Prophet's death  should not prevent him from fighting in the way of Allah, for Allah was alive. Saying this Anas rushed at  the infidels, and was martyred after having received seventy wounds.   

 http://www.witness-pioneer.org/vil/Articles/companion/05_umar_bin_al_khattab.htm 

  
 however, he had no problem killing defensles people: 

 t is recorded on the authority of Abul Aswad that two persons of Madina who professed Islam but did not  realize its full significance had a dispute among themselves and took the matter to the Holy Prophet for  decision. After hearing both the parties, the Holy Prophet gave his decision in favor of one person. The  other person felt dissatisfied, and at his instance both of them went to Umar and the aggrieved person  lodged an appeal with Umar against the decision of the Holy Prophet.  

 After hearing both the parties, Umar said to the man who had filed the appeal: "So you are dissatisfied  with the decision of the Holy Prophet, and want me to reverse his decision."  

 "Yes, that is so," said the man  

 "And are you a Muslim," enquired Umar.  

 The man said, "Yes, I am."  

 Thereupon Umar said, "Wait for a while. I will soon give my decision that would satisfy you."  

 Umar went inside the house and brought his sword. With the sword Umar smote the appellant saying,  "Woe to you, you regard yourself a Muslim and yet choose to appeal to me against the decision of the  Holy Prophet. You are an infidel, and the penalty for your infidelity is death."  

 The other man went to the Holy Prophet and complained that Umar had killed his companion. 

 http://www.witness-pioneer.org/vil/Articles/companion/05_umar_bin_al_khattab.htm