Question: When David tells Betty of what happened to Bruce as a child, David said that he wanted to cure Bruce but was stopped. If his original plan was to cure Bruce, then why does he suddenly want to absorb all the gamma energy out of Bruce and then kill him?
Answer: He wasn't trying to kill him, he was using his son as a guinea pig. Experimenting on him, poking and prodding him, altering his genetic structure. That trying to cure him was all crap.
Then why try to kill Bruce when he was a child?
Answer: It's called lying. He was an egotistical mad scientist, who was paranoid and obsessed with proving his work valid. He was willing to do anything to achieve his goals.
David wasn't exactly lying. He attempted to kill Bruce when he was still a child but ended up killing Edith instead when she tried to stop him. General Ross was even there when David was taken away so there is some truth to what David said to Betty.