That means the girl killed 4 innocent people just for the anger on her boyfriend. How cruel she is? I think her boyfriend done breakup for such kind of nature of her.
I know if we think about it then we can't trust anyone but we should not think too much about it, we can't trust anyone and take food outside.
I think her plan had been to kill just the boyfriend, but he ended up sharing the entire meal with his friends and that killed all five of them.
And yeah I understand that thinking about it too much might start to mess with our brains, sometimes you just can't help it.