It may never work, because not everyone wants to be together, but we ought to try even so. Division is damaging. Yesterday, it cost someone his life in horrific circumstances. If we react by becoming ever more divisive, that will in the end lead to yet more deaths, yet more violence and misery. The reactions of those who brand a whole community, or a whole society, are different and to be feared because of the actions of a few are understandable but completely the wrong approach. It solves nothing. Just more divisions. Yes, we should try to unite.
So how about we start here, and not argue amongst ourselves?
In answer to the second point. We don't yet know exactly what was known to the Security Services about these people. It may have been very little, in which case there was nothing more that could have been done -- you cannot arrest people and lock them up on essentially no evidence. It may have been a lot, in which case why was nothing more done? Hindsight is a blessing and a curse -- now we know that the two men probably should have been arrested beforehand, but we didn't know that yesterday morning. I just hope that our Security Services didn't miss a genuine chance to do something about it.