You are not only assuming the SG was killed, you are assuming that she was killed by the same SK that killed the 4 victims.
There is really no real evidence to support those assumptions.
As the last person who saw SG alive, Mr. Coletti said, she could have just drowned.
As a matter of fact, the MO of the SK is totally inconsistent with the way SG was hired. Having SG's driver deliver her to the John's home is not something a SK will do.
If JB & co. had wanted to kill SG, they had 3 entire hours to do it, then what would they do with her driver, who was standing outside waiting for her?
It just makes no sense.
Her refusal to leave with her own driver, someone she knew and trusted is entirely her own doing, and nobody elses. We don't know what kind of drugs she was on and what she was really thinking.
From the testimony of everyone who last saw her, SG was delusional, irrational and probably was hallucinating.