It is sad, isn't it?
I'm sorry if I were one of the friends he mentioned this to a handful of times I would've started thinking this wasn't really a joke and more of an omen that he was really seriously thinking of hurting/killing this girl. There have been many instances of people who talk like this and others who think its a joke and then something really awful happens and they go through with doing something truly awful and people end up dying...like in the case of the Columbine shootings.
The friends should be reprimanded for not telling someone after he said this to them a few times. It should have indicated he was really considering it and that it wasn't just a sick joke. ...and tbh the free breakfast shouldn't have anything to do with it. I wouldn't chuckle a bit if I knew the outcome of someone killing someone else was a free breakfast.
For approximately the last year, I have been listening to my best friend explain to me of his plot to rob the local bank. It is actually rather intelligent and well thought out, and he has refined it continuously. Should I report him as a would be robber?
A few years ago I had a teacher that took a disliking to six students in particular(myself included), she would attempt to humiliate and embarrass us. As well as use the slightest excuse to give us detention. We used to joke about how much we hated her, and intended to kidnap her and drive her off a cliff at the end of the year. Should we be arrested as a terrorist cell?
Have you heard of the term paraprosdokian? It is basically a sentence, that when you hear it in full makes you re-examine the beginning of the sentence
for example, "cutting up onions brings a tear to my eye, he was such a good dog"
Because we know this story ends in tragedy, it is easy to look back on those conversations as an ominous foreshadowing of the impending murder. If he had not killed that girl in the end, no one would ever bat an eyelid at that kind of conversation, not in today's world imo