From the BBC:
The threat is growing, MI5 is stretched, some of its capabilities are at risk.
All of that means something is likely to happen. That was the bleak message from [head of the UK’s MI5] Andrew Parker.
While I disagree with Mr. Parker’s assertion that the security services need more powers to intercept and monitor all communications (which, in the UK, they pretty much do already, so I’m not sure how they’d increase that ability), I do agree that it’s unreasonable to expect 100% safety or 100% success in stopping bad guys from doing bad things.
Thus, as always, the answer is to be reasonably prepared to take care of oneself in various situations until the cavalry arrives: in this case, having a gun and the training and will to use it effectively if the need arises while hoping that one would never face such a situation.
Consider, for example, the photos taken of the killers in Paris by citizens sheltering on nearby rooftops. Had one or more people on those rooftops been armed, even with handguns, they would have been able to fire from an elevated position on the unsuspecting bad guys. At the very least, this would have caused confusion on the part of the bad guys, distracted them, and slowed them down — hopefully until the police could arrive.
Alternatively, if directly confronted by armed bad guys intent on murder, being armed gives one at least a fighting chance of surviving the encounter. Success is not assured, particularly when it comes to defending oneself from heavily armed attackers who had the element of surprise, but it’s better than nothing.