The most terror can be created by hideous attacks on the most innocent people, who are usually the easiest targets.
Civilized people have a difficult time trying try to understand and rationalize it in terms of "reasonable" targets, some reason for this. The actuality is something worse than most want to contemplate.
In almost every one of these attacks, the killers are learned to have been known to the police, who kept too much distance, or failed to pick them up. With over 1,000 ISIS and Al Qaeda known to have returned to the UK, this mass murder should be no surprise.