Mirthful murderers

I was discussing current affairs with some friends last night. One noted that there had recently been uproar and revulsion in the US in reaction to a couple of students in Pennsylvania who posed for Facebook pictures dressed as victims of the Virginia Tech gun massacre.

One's initial reaction to this seems to be to concur with the revulsion (or at least mine was). But then the same friend noted that another of our friends attended a Hallowe'en party this year dressed as Harold Shipman. I recalled back to when I first heard about that, and I reacted with mirth.

But when you analyse the two situations with cold logic, what is the difference between them? Is it simply that different amounts of time have passed since the killings that brought respective notoriety? And if time is the key factor, how long do you have to leave it before revulsion turns to mirth? Is it a sliding scale? Or does the severity of the crime have to be thrown into the equation too??