Guilt by association?

There was some effort made at the time of the killing of an MP -- Jo Cox-- to link the crime to the Brexit vote and it has been renewed today in the Guardian (one of several pieces in fact), following the conviction and sentencing of the murderer

The argument has a classic form. The judicial facts about the case, such as they are, and the sentence are reported in some detail, and the Brexit angle is inserted into the main narrative as if it is also authoritative.

The person who killed Cox (Mair) had a long history of interest in violence and in right-wing politics extending over some years. He was a 'slow-burn' terrorist.

Yet the Guardian wants to add that :

[Although] The seeds of the hatred that drove him to murder his MP, Jo Cox, appear to have been sown years earlier, when he began to acquire the means to kill. They germinated during the febrile countdown to the EU referendum.

.... Just hours before the murder, Ukip unveiled its infamous “breaking point” anti-immigration poster

There is no actual evidence reported during the court proceedings to support the 'Brexit germination' line as far as I know, although it is perfectly possible of course. The defendant said nothing about his motives  -- indeed, he said nothing at all. The line was widely asserted at the time, in several variants -- even if Brexit was not an actual cause of the murder, the nasty 'atmosphere' created by the referendum itself was responsible

The whole area of Yorkshire where the murder took place was known as a location for right-wing activism, apparently:

police discovered 54 homemade bombs and a dozen firearms at a house in Batley. The occupant, Terence Gavan, a BNP member, was jailed for 11 years after admitting a series of offences under the Terrorism Act 2000. The court heard that Gavan showed “strong hostility” towards immigrants.

but then 

It appears that Mair, however, had little to do with such groups, perhaps because he was so reclusive. He preferred his relationships with the far right to be long-distance affairs.

So far nice and 'balanced' . The Guardina also reports

Mair himself claimed to be in need of treatment for mental health problems

Turning to the Brexit germination 4line, though, The Guardian puts more sentences side-by side to suggest a connection:

It appears that Mair also woke up one day and decided he was going to do it.

Cox was a vocal supporter of the remain campaign. Her constituency is a place where anti-European feelings run high, and each day during the referendum campaign, Mair was surrounded by red and orange Vote Leave notices. St George’s cross flags fluttered from windows. His “death to traitors” outburst during his first court appearance shows he regarded Cox as one of “the collaborators”, the white people who had betrayed their race.

 ...

He did [the murder] according to the eyewitness, while saying: “Britain first, keep Britain independent, Britain will always come first.”

Finally, he yelled: “This is for Britain.”

OK -- it is all plausible enough, but it is just asserted that Brexit and racism went together, as usual. The Guardian couldn't just report the murder, nasty and unsettling as it was, without trying to hook it up to their post-Brexit line, whether it actually fitted or not. The article makes no attempt to assess 'objectively' the importance of the Brexit germination factor, and offers no counters, unlike the discussion of all the other factors. Cox has to be not only a victim, as if that is not bad enough, but a martyr.