And the racists and fascists have been quick to start shouting and swearing angrily about how disgusting it is for people to be making political capital out of her death, for daring to talk about why she was killed.
I mean, consider Larossi Abballa's murder of a French police officer and his wife. Abballa explicitly made a statement where he claimed to be acted on behalf of the Islamic State, and the Islamic State's press agency released a statement praising him as an Islamic State warrior. But I wouldn't take that as demonstrating that all Muslims everywhere are advocates of murder or assassination or terrorism, and I expect you wouldn't either.
So we have Thomas Mair murdering a British MP. He made a much less clear statement that some people have interpreted as support for Britain First; Britain First has denied his membership and condemned his actions. So the link between him and Britain First, or between him and the anti-EU point of view, seems weaker in the first place. But even if it were equally strong, saying that all Leave voters are racists or fascists is the same kind of overgeneralization as saying that all Muslims are supporters of terrorism.