What were Diana Moon Glampers' motivations in "Harrison Bergeron"?
The thing about authoritarian governments is this: the leaders all believe that they are doing the right thing for their respective countries regardless of the negative consequences. Diana Moon Glampers is no different. This idea of everyone being "equal" would be, to most at least, a desirable trait and Diana Moon Glampers is enforcing that trait. So, even though we might look at Moon Glampers's ideas regarding "equality" as warped, she probably sincerely believes she is doing the right thing.
Throughout the story, George Bergeron, the father of the title character, explains why equality is important. He sums it up here when his wife suggests he temporarily removes some of his handicaps (the devices he has to carry to make him "equal" by lowering him to others):
"If I tried to get away with it ... then other people'd get away with it and pretty soon we'd be right back in the dark ages, with everybody competing against everybody else."
This thought process, in addition to the society being prohibited from thinking, is why there is not outrage when Moon Glampers walks into the studio and immediately kills Harrison and the ballerina who ripped off their handicaps and were "leaping" into the air. But, according to this society's thought process, Harrison and the ballerina were in the wrong. Moon Glampers killed two people who tried to destroy the current social order, which seems to have brought a sense of peace to this society.