He had a right to remain on the property initially but only until the representatives of the owners of the property, in this case the COS, revoked his right to be there and and asked him to leave. You have a "right" to remain on private property only so long as the owner permits it. Once the owner revokes permission, you are trespassing if you remain. You have to leave.
The way it normally works is as follows. The COS asks someone to leave, and tells them that if they don't the COS will call the police. If the person agrees to leave, there is no need to call the police. And if the person agrees to leave, you certainly aren't going to wait for the police to escort him out.
The 16-year-old apparently agreed to leave and was being escorted off the property. At some point, he pulled a knife and stabbed people, killing one.
I believe any attempt to blame the SO security guards in this case ignores the evidence of the alleged perpetrator's intent and willingness to commit violence -- i.e., carrying "a 25cm [i.e., almost 10 inch] “very sharp kitchen knife."" See
https://www.news.com.au/national/ns...s/news-story/661dd8a201bfabee9e156e8c03345376
The fact he carried such a knife provides evidence that he came there with the intent to hurt someone, probably his family member.