There is a big difference between how you think MI-5 operates and how it actually operates. MI-5 agents spend most of their time in meetings and compiling reports and interpreting information than conducting secret missions and conspiracies. That's what assets are for. Now I would be very surprised if Britain First, the BNP et al didn't have people who were passing information on to MI-5, but its not really the kind of thing they do, targeting 'mentally unstable' people to murder politicians. I accept it might very well be willing to manipulate people to do away with dangers and threats to the UK.

That's not really how MI-5 works. Most domestic intelligence agencies don't kill their own citizens, because that's the kind of order, that causes agents to start leaking or breaking cover to expose them (remember most people join MI-5 to protect people, much like people join the army - Ordering the army to attack its own citizens, generally ends badly as well).

Whilst I don't doubt that MI-5 has had its hands in the pies of a few murders, especially in NI, killing an elected MP in the UK isn't their bag. Especially using a mentally unstable asset, who could bring the whole house of cards down.

"One Nation Under God, has turned into One Nation Under the Influence of One Drug"
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Oswald wasn't mentally ill though - not in the disordered sense - Of course I suspect he was part of some kind of conspiracy, but nothing really dramatic. Also Kennedy was a president of the USA, who had made some rather serious enemies in the Soviet Union (Cuban Missile Crisis) and Cuba (Bay of Pigs). I wouldn't have put it beyond reason, that the soviets or elements in the soviets had recruited Oswald during his time in the soviet union.

There were certainly factions in Cuba and the Soviet that had an axe to grind over those two issues (not to mention the repeated attempts by the US to kill Castro). Also Organised crime in the US had some serious issues with Kennedy (though whether the Mafia or families of the time would have sanctioned the murder of a president is questionable).

Where as Muir seems to have a history of psychiatric illness, which makes people really unreliable as assassins, and his target was someone no one really knew or had heard of prior to being murdered.

My suspicion is that Oswald was the trigger man, and that the Cubans and/or soviets pointed him towards 'doing it' and he wanted 'in with them', and those involved were out of the country long before the shots were fired.

"One Nation Under God, has turned into One Nation Under the Influence of One Drug"
[Link]

And very conveniently topped before he could come to trial, by someone who himself was terminally ill.

First page of Google

Mentally ill is probably somewhat unfair, given that on the whole the mentally ill are more at risk from normal people and themselves, rather than a risk to others.

I don't think it was a random attack, I think its likely Muir planned it and in his mind, saw himself as firing 'the first round' in some kind of revolution or something similar.

Whether he cared who the target was specifically, is a question of debate. Likely as not, it was aimed at what he considered 'the enemy' and 'Remain' had become that in his mind - and Jo Cox one way or another came to symbolise that within a growing delusion.

It wouldn't be a random outburst, given it seems he may have manufactured a gun etc. But a disordered mind can be capable of quite rational behaviour and organisation.

"One Nation Under God, has turned into One Nation Under the Influence of One Drug"
[Link]

Occam's Razor.

Although the most likely is by no means the answer every time.

Life is stranger than fiction.