The terror attack in London

has left me questioning yet again, "Why?"
Terror attacks like this don't make sense to me...well, no terror attack does but these make even less sense.
Can someone please explain why it makes sense to go out and kill innocent people going about their daily business? Why kill people who are not doing harm to anyone else? Why get yourself killed?
It makes no sense.
A long time ago now I answered the phone in our home and, before I could say anything, a terrified voice at the other end said, ".....my Dad is trying to kill Mum."
No, it wasn't a joke. We lived in a "soldier settlement" at the time. One of those areas set up by the government where returned servicemen were given parcels of land to farm. It was a well meant but crazy scheme, particularly in that extremely isolated area. The rate of mental illness among the men was particularly high. 
On this occasion the farmer in question was chasing his wife across a paddock (field) with a red hot poker in his hand. He thought she was the enemy. 
The Senior Cat didn't hesitate to believe the child. He made a couple of phone calls, the farmer on a neighbouring property went with his teenage son and restrained the man until other help arrived and the poor man was taken off for a spell in psychiatric care. He had done no physical harm and no charges were laid. But of course it harmed his family. The marriage eventually broke down. It wasn't the only incident while we lived there but it was perhaps the most dramatic. 
It affected me too. I was only a young teen at the time.
I look at the news now, at the teens and the children going through the most horrendous experiences. Their apparent calm is not real. Underneath they have to be living in a constant state of fear as well as coping with a lack of food, of shelter, and of all the things they should have as a right. Their experiences and that of the young boy on the farm are far worse than the one I had.
I don't understand why any human being would deliberately put another human being through something like that. It makes no sense. What sort of "god" do these people believe in?