I do think gun control needs to be improved but the problem is that a mentally unstable person can suddenly go over the edge and easily get a gun illegally or legally.
I disagree. I wouldn't have a clue where to get a gun given time let alone in the heat of the moment.
I realise this would be different in a country awash with firearms but not in England. Of course there are a few 'gangsta rude boys' who claim they can get one but it's certainly not the case that in the UK your average Joe can 'easily' access a firearm at a moment's notice.
I'm pretty sure I'm right when I say the majority of people in the US own a gun and that most of them are law abiding citizens.
You are making the same logical fallacy I touched on earlier. It is not the case that you have criminals and 'good guys' and neither can cross over into either camp at any moment.
I'd bet most of the people who commit mass gun slaughter using legally owned guns were 'responsible gun owners' prior to them going mad too.
No one can guarantee that at no point in their lives will they ever suffer a mental breakdown and use their weapon to kill innocent people.
I see no problem with people having a means of defending themselves.
So why shouldn't they own hand grenades and rocket launchers then?
Also it's a circular argument. If a career criminal (let's say a mugger) knows his targets are likely to be armed he just buys a bigger and powerful gun than them. If he knows they are not armed he may just go with a knife or other weapon that would get the job done but not risk a mass killing spree.
He also has the advantage of having his gun drawn first. If someone pulls a gun on you and you try to pull a gun out they'll probably shoot you before you even have the chance to pull the thing out of your holster. Whereas if you just give them what they want and let the police deal with him later you've got a decent chance of surviving. What is better, having some material objects taken from you and surviving OR getting into a gun fight where you might keep your stuff and end up killing another person or (more likely) being killed yourself?
The police spend years training to be a good marksman, the idea that your common man with no professional training would, in the event of a shooting spree, turn into Magnum PI, roll over a car bonnet and take out all the bad guys with his pistol are solidly located in the world of fantasy. Most people would run like hell despite being armed than enter into a shootout.