I think I misread it last year.
They were revealed to be vampires right after debut but that didn't mean all of them killed Shikacharu together.
https://www.youtube.com/watch?v=JO5FNQTMGtMThe culprit is still one of them and it was never canonically revealed, albeit some mystic hint materials like above. But I'm finding some fan theory crafting about which of them killed him.