Quandary

At what point does the brain draw the line between Shadenfreude and empathy?

I'll pause here for a quick lesson:
Schadenfreude is a German word meaning 'pleasure taken from someone else's misfortune'. It has been borrowed by the English Language and it derives from Schaden (damage, harm) and Freude (joy). There is an English equivalent which is epicaricacy which is derived from the Greek epi (upon), chara (joy), and kakon (evil), but it doesn't feature much in the language anymore and you will be hard pressed to find it in a modern English dictionary.
Another phrase with a meaning similar to Schadenfreude is "morose delectation" ("delectatio morosa" in Latin), meaning "the habit of dwelling with enjoyment on evil thoughts". The medieval church taught that morose delectation is a sin.
In modern days we have been able to map images of Shadenfreude at work in the brain, and it is easy to feel the effects of the serotonin realeased, just think back to the news article of Captain Dan the Dwarf who accidently super glued his vacume cleaner to his groin.
I should mention at this time that we are balanced by the buddhist concept of mudita, "sympathetic joy" or "happiness in another's good fortune," which is an example of the opposite of schadenfreude.

Back to my orriginal questions:
With the fact that we can derive joy from others misfortune what makes our brains draw the line. I've posted several news articles now that have related to people either being in pain and embarisment like Captain Dan the Dwarf or just straight out dying. Deputy mayor of Delhi, death by monkey attack. 60 yr old woman killed by rambunctions pet cammel. Drunk guy nudes up and hops in with a bear, [surprisingly] gets eaten. Incredibly fat guy, forklifted to hospital.

All of these things highlighted schadenfreude for me, I found them hilarious. I admit I felt no simpathy for the dead or the embarised. I didn't know them and the situations were just too easy to mock.

I little while after I posted these items Salem posted a news article about a zoo keeper who went to feed a bear, the bear much like the above mentioned cammel got a little rambunctions and killed the zoo keeper. For some reason my schadenfreude didn't show, my sence of epicaricacy escaped me. I was horrified by this article, every sentence I read hammered home a feeling of revulsion and even sorrow for the poor man killed.

Why though did I feel this way. I don't know any zoo keepers, it certainly wasn't the action as I laughed at a simmilar action with the cammel. Could I not laugh at a bear related attack since i'd already posted one with a bear? Does everyone only get one? Was my schadenfreude just out of balance with my mudita? Did I need to see someone have good fortune before I could laugh at the zoo keeper?

Interestingly I mentioned this to Salem and he told me that he felt the same way about the fat guy post, he just found it repugnant.

This isn't something that I find disturbing just curious. Have you ever been in that situation, you see, read or hear something and you know others are amused by it but it just seems vulger to you?