"Ellie" killed her son because she knew he would blow the place up. She can't have that happen because there's one person missing. Desmond.
Desmond has been there and he knows it. If Daniel had blown up the island what would have happened to the world with Desmond knowing ?
Besides Elloise told Daniel that if he returned to the island it would cure him. He's not going to die. If he does that will set off Jack to find a way to bring his body back to 1954 thus bringing him back to life and where he ( and Ellie and Widmore) started from.
Desmond has been there and he knows it. If Daniel had blown up the island what would have happened to the world with Desmond knowing ?
Besides Elloise told Daniel that if he returned to the island it would cure him. He's not going to die. If he does that will set off Jack to find a way to bring his body back to 1954 thus bringing him back to life and where he ( and Ellie and Widmore) started from.