LennStar I think that may be a too generous reading of the threat. If Trump were to win the election, he would also "not concede the election." And even otherwise, the only thing an individual voter fearing for their life could do to reduce the risk that Trump loses and fails to concede is to try to make sure Trump loses in enough of a landslide that there is no ambiguity. So this still looks like a threat of violence with the objective of influencing how the person who was threatened votes. And that's woefully bad.
Now generally election related threats of violence tend to come from individuals who may be struggling with low grade mental illness and/or big challenges in their personal lives, not organized groups. A good example of the type who seem to both make threats and sometimes carry out those threats is that guy who tried to shoot a bunch of senators at a baseball game in DC back in 2017: https://en.wikipedia.org/wiki/2017_Congressional_baseball_shooting#Perpetrator But another equally good example is the guy who shot the congressperson in Arizona close to a decade ago: https://en.wikipedia.org/wiki/2011_Tucson_shooting#Perpetrator (I'm including two so it doesn't look like I'm trying to focus on attacks on republicans or focus on attacks on democrats). When things are going badly in your life it is easier and easier to look outside yourself and blame everything on those other bad people.
That doesn't make the threats any less scary (or less dangerous) for those individuals targeted. It just means particular threats tend to be localized and not big enough to effect the overall outcome of the elections.
The MAGA Bomber would be a better, more recent example than the guy who shot Giffords (which, by the way, part of that hand-written death thread on the door was that he would "finish the job" on Giffords).
Do these people clearly have mental problems? Yes. Does that mean that everyone else is absolved of all responsibility? No.
There's a whole infrastructure on the right of intentionally manufacturing fear and hate propaganda to radicalize people, up to and including the president telling his supporters that the only way he will lose the election is if it's stolen via voter fraud, and telling "the second amendment people" to "do something" about Hillary if she wins, and pastors telling people to rise up and kill "the Marxist BLM protestors who are trying to destroy the country", etc. They push extremist fear and hate rhetoric to intentionally radicalize their people and divide the country, and then some of the less-stable people are pushed over the edge, and this is the result.
Now you could argue that the "Lamestream Media" is doing the same thing by questioning whether Trump will step down if he loses. However I submit that that's not at all the same thing, given that Trump repeatedly "jokes" about not stepping down, or staying for a 3rd term, or being dictator for life, etc, and refuses to actually say that he will peacefully leave office if he loses. That's just accurately reporting what the president says at that point, not intentionally trying to manufacture divisive hate rhetoric.
I will also point out, given the new news from US Intelligence that Iran was behind the "you'd better vote for Trump or else" emails that people in Florida got, that we should be suspicious of any case where the cops don't yet know who the perpetrator is. These "If Trump refuses to concede if he loses you'd better have your home insurance current" letters that Trumpers are getting are awful, yes and absolutely, I will agree 100%. But it's also
possible that they are actually a false-flag, or the result of some foreign government interfering. In the two examples I linked they already have the guy responsible in custody, so that's not quite the same thing. In this election I think it's probably fair to assume nothing until we have official word from relevant authorities or have first-hand information.
