At the simple level - which is also entirely valid - the responsibility apparently rests with the accused shooter, the 52-year-old Englishman, Thomas Mair.
At the more complex level, I would argue, it is probable that some of the responsibility rests with Donald Trump or, one might say, with the "spirit" that Trump has unleashed into the world with his campaign.
Is that connection plausible? Is that accusation fair?
For starters, a good deal of the discussion of Mair's actions has connected Mair's actions - killing a pro-EU member of parliament -- with the increasingly inflammatory anti-immigrant rhetoric of the "Brexit" side of Britain's current intense debate on whether to leave the EU.
That connection is made doubly plausible by the fact that Mair is reported to have shouted "put Britain first" at the time of the shooting, and that he subsequently gave his name in court as "Death to traitors, freedom for Britain." A kind of white nationalism - which had become increasingly vocal in the political environment surrounding Mair -- seems to have motivated Mair's attack on Cox.
Had there been no EU referendum, or if the debate over "Brexit" had stirred up less bigotry and anger, there is a reasonable possibility that Mair would not have made his lethal attack. It is entirely plausible that the intensifying force of bigotry rising around him pushed him over the edge.
(It seems plausible also to think that the British themselves have drawn some such connection, for it appears that in the wake of this crime, the momentum on the referendum has swung back from "Leave" to "Remain." That swing in public sentiment suggests that -- in the eyes of a segment of the British population -- the assassination of Jo Cox discredited the "Brexit" cause on account of its role in strengthening some ugly passions in the public arena.)
If it is reasonable to see Mair's as influenced by the emboldened white nationalism in Britain at their moment of decision, is it also reasonable to imagine that the rise of Donald Trump in America could have similarly influenced Mair?
There are two ways that could plausibly have happened -- one through Trump having an impact on Mair directly, the other through Trump fortifying the white nationalist/supremacist forces in Britain, which in turn influenced Mair.
Mair himself has ties with American neo-Nazi groups, and such groups in America have been quite vocal about how encouraged they've been by Trump's success. Since Trump has emboldened these groups, surely Mair may have been emboldened likewise.
Trump's emergence -- from a supposed political joke to clear front-runner to the presumptive nominee of one of America's two major parties -- has been huge news not only in this nation, but all around the world. That certainly includes Britain. We may recall that a popular petition in Britain led their Parliament to debate a resolution that would have Trump from their nation. So clearly, the image and message of Trump has been an important part of British consciousness.
If Trump has encouraged an emboldened America's racist/nationalistic fringe --as he has -- is there any reason to believe that he wouldn't have a similar impact on similar groups in Britain? Would not the rise to a position of great prominence of someone expressing a spirit akin to their own fortify the forces of racism and bigotry in our closest ally as well?
We cannot know whether, has there been no Trump, Jo Cox would still be alive. And in any event, there is no question here of Trump having any legal responsibility. But the human world's densely interwoven fabric of causes and effects means that a given action can be the fruit of a variety of causes. And as every action has many sources, moral responsibility for those actions properly gets spread around does spread around.