There does in fact appear to be evidence that Israeli forces, with Ariel Sharon in command, colluded in this massacre. This includes evidence from Israeli soldiers who were present.
The Israeli Kahan commission found that, while the actual killi9ng was done by the Christian Phalangist militia, Israeli military personnel were aware that a massacre was in progress without taking serious steps to stop it, and that reports of a massacre in progress were made to senior Israeli officers and even to an Israeli cabinet minister; it therefore regarded Israel as bearing part of the "indirect responsibility." In particular Ariel Sharon bore "personal responsibility", should be dismissed as Defence Minister and never allowed to hold public office again.
This did not prevent Sharon from subsequently becoming Prime Minister of Israel.