We are never given the ability to know what’s in the heart of people around us. As such, we should not judge someone based on his or her past. A sinner may repent. A non-sinner may turn bad. Only God does have the ability to judge in the most fair and knowledgeable manner. That’s not our duty at all. It is enough and recommended that we always judge ourselves and strive to be a better person from time to time. This is Sunnah. Furthermore, every Muslim knows ‘Umaar was the 2nd caliph. Who was he in the past? He was someone who wanted to KILL Prophet Muhammad. I believe this sets one of the best examples regarding this matter.