Darts could kill someone, just as a lot of other things we don't normally associate with death could if they hit the right spot. I certainly wouldn't want one flying at me, put it that way! The argument that a gun in the wrong hands is a bad idea is stating the bleeding obvious. The same could be said about cars, which are killers every day of the week, but we don't ban cars because, say, a man with a spotless criminal record gets behind the wheel one day and goes on a killing spree (as indeed one did quite recently - albeit a van, if memory serves, and how hideous was that) or people act like idiots on our roads every day (which they do).
The primary purpose and use of guns in the world is to kill people or to provide the threat of killing people. Outside of a very controlled range scenario, there are trivially few legitimate needs or purposes for a gun in the UK.
As it stands in the UK, if a criminal goes out with a gun then he's escalating any situation and will go away for a very long time, even if he's just posing with it and not intending to commit a violent crime, sentencing guidelines mean that it adds at least 5 years to the sentence of any crime. Burgle a house without a gun and you'll probably get away with it with the police not even bothering to do much but give a crime number. Burgle a house and threaten the owner with a gun and you'll get the undivided attention of a large group of very serious policemen. If I had my way, anyone who committed a gun crime would get a mandatory life sentence never mind the 5+ years that get added now. In the US, many criminals carry guns because there's a chance their victims will have one.
There are almost no situations in the UK where someone can accidentally be shot with a legitimately owned pistol or rifle, unless the owner has broken the law or licensing conditions. You won't find a bullied kid stealing his dad's rifle and going on a killing spree at a UK school unless the owner has broken the law or licensing conditions. You won't get a toddler accidentally killing himself because he thinks the gun is a toy. That's the sort of stuff that Farage risks bringing back if he ever got his way with allowing handguns back in general public use.