Given that the perpetrator was known to the Police, it should have raised a flag when the rental company imputed his driving licence details...
If the driver had merely had an episode and crashed into the pedestrians then one would reasonably expect to claim against their insurance 
The fact that the driver was a psychotic loon and is no longer present should make no difference IMHO.
Perhaps, but that perpetuates the view that anyone who encounters any setback in life is somehow
automatically due financial compensation from
someone and, if they can't chase the person responsible, then they need to find another
someone loosely attached to the incident and chase them instead.
In the case that negligence or an accident directly caused injury, then that's (perhaps, depending on the circumstances) fine, but insurers don't cover against terrorists, so should we be saddling a company who hired out the vehicle lawfully and in good faith with the costs?
It seems that we are (re-)entering a period where a few of us must experience loss of life or a reduction in the quality of life as a cost of maintaining the society in which we choose to live. That's happened before in two world wars, throughout the troubles in Northern Ireland and in countless other conflicts. One of the principles of that society is that we don't abandon those people caught up in conflict, and that they are entitled to heathcare and benefits to help get them back on their feet.
I can't put myself in the position of that survivor but I really hope that, in his situation, I'd be able console myself with the fact that, although unfortunate to be caught up with it, I ended up as one of the fortunate ones, relatively speaking.