His plan was destroyed. Elizabeth is lost. To Sao Feng, that pirate Lord who tortured Will with near drowning, and who looked at the woman he loves with such animalistic lust. Will's only hope to save his father, by gaining control of the Black Pearl, has failed, and the fastest ship known to sail is now in control of the most selfish excuse for a pirate ever to return from Davy Jones' Locker.
Will's last hope is to complete his betrayal, and lead Becket and Jones to Shipwreck Cove itself, there for one final barter for the lives of Elizabeth and his father. And right now, he has no care at all for how he does it.
Most of the men crewing the Black Pearl now were never loyal to Jack. They were Sao Feng's men, given over to Will as part of the plot to gain the Black Pearl. Will's surprised Barbossa and Jack allowed them to stay - but then, they still needed some men to crew the ship. Nevertheless, for a while Will considered them his men, and he feels no remorse at all as he sneaks up each one he finds alone, kills him cleanly with his father's knife before lashing the body to an empty keg. Haulling it overboard, Will trusts the scavenging sea birds will be enough to lead Beckett on their trail.
Everything Will has ever done in his dreams with Elizabeth, to Sao Feng is doing forcibly, at knife and gun point in his every waking thought. Will almost takes pleasure in killing that man's crew.