<PERSON> was not cursed to begin with but once he persuaded <PERSON> to ally himself and not kill <PERSON>, he picked up 4 coins and threw back 3 to dramatically say "every" (throws down coin) "last" (throws down coin) "One" (throws down coin). This was a trick to make It seem like he replaced all coin directly back to the chest without really taking any BUT he had a fourth coin and the camera shows him slyly palming It and extending his fingers to indicate an empty hand, classic gamblers trick. He then stepped away from the chest which In turn would curse him. Another factor to this plan working Is that the rest of the pirates loathe the curse having been under It for so long, It probably didn't occur to them that someone would deliberately allow themselves to be cursed just to be temporarily unkillable.
The main reason <PERSON> did this Isn't necessarily that he was worried he'd die In general but he knew he'd have to fight <PERSON> who has been suggested to be even better than him at swordplay.
To answer some above suggestions that <PERSON> was Immortal since before then, no he wasn't.
One of the crew saying "you're supposed to be dead" was In reference to him being left on an Island to die but only <PERSON> knew It was a regularly stocked point for merchants. 516
No doubt knowledge from his days working for the East India Trading company and <PERSON> Is known to keep his cards close to his chest.
The apple he ate wasn't finished, <PERSON> took It before he had <PERSON> locked up. <PERSON> Inspected the apple and angrily threw It away. This, coupled with the fact <PERSON> rummaged through the apples on the table suggests that <PERSON> Isn't In the habit of poisoning all of the apples, only the top ones and <PERSON> knows his methods having been crew mates and friends.
Additional note: <PERSON> and <PERSON> are solid friends but It Is likely the curse made <PERSON> unusually ruthless to get what he wanted. He said he felt nothing, what would this say of his conscience? There probably wouldn't be one, hence why when he was alive again he was far more amicable with <PERSON>, had his honour and "sense of fair play". 417