Hancock's 'betray' wasn't open enough since she was fighting Whitebeards too and the circumstances often got confused (people mistook her love-shock for Luffy fighting her, and her pursuit of him at the end as actually doing her job instead of trying to aid him), she's still in.
Moria didn't betray them per se, but he was fired for weakness.
So all in all, three empty slots.
what Nephthys said...
as for dudes being offed, we've been shown a couple albeit it happened in the past, Sabo being blown up along with his ship, and Aokiji freezing Jaguar D. Saul to name a few...
I also read somewhere that Zoro has killed prior to joining the crew...
also not sure if Mr 2 was killed by Magellan off panel...