Adding my 2 cents.
It's very much both, they know his secrets, so they have to be disposed of to protect the lie. The death must look like an accident as Ber noted, because of potential investigations.
I lean towards Hypnosifl's explanation that maybe the name Ozy was chosen by Moore and I like his rationale of Ozy being confused at the end and asking if it works out in the end. It's more of a nod to the reader... but I like the other way too. I definitely think him killing the staff was practical first and foremost, and he just used the parallels of Pharoahs taking their whole staff with them as an excuse for his crimes. Cuz he is a massive douche.
Thing that doesn't quite make sense to me is why neither Ozy nor Manhattan seemed to think Laurie or Dan should die. Like, I get that they are kinda friends, but Ozy killed ANYONE who might ruin his plan. He didnt hesitate to kill many ppl close to him, so honestly why risk it with Laurie and Dan?