This is a very tough name to appraise. It really doesn't have a strong commerical appeal, but it has value as a "Novelty Name". Some of the most overrated names are names that are actual words/dictionary names that don't have obvious commercial uses. This is one of those. And since it is a .net, it also hurts.
However, even with it being a .net, this is a nice brandable term, that is also easy to spell. There are probably a wide-range of end-users, as well. These end-users could be anything from someone putting up a site about serial killers, to something like a horror-movie site. There are possibilities out there, but the problem is in finding these people without turning it into a full-time job.
There are probably a number of sites with the word Killer that are at the .net extention, too. They could be possible buyers.
$500 is the value I put on it. It will take a lot of luck and/or hard work to get much more than that, and I am not so sure this is the type of name you could fire-sale quickly.