United went ahead through a well-taken Robin van Persie strike in the 27th minute at Old Trafford, before Radamel Falcao doubled the advantage five minutes later.
A Wes Morgan own goal made it 3-0 at half-time, but United were unable to add to their tally after the interval, with Marcin Wasilewski scoring a late consolation for the visitors.
The win helped the hosts avenge the 5-3 defeat they suffered at Leicester earlier in the season, having also led that game 3-1, but Van Gaal says his team became too conservative during the second half.
"I asked them to kill the game, because when you remember the game at Leicester, we did not kill the game," he said. "But I think we killed the game too much.
"We played more balls backwards and wide than forwards and we have to learn that you keep patience and don't take risks.
"But when the pass is available because of the options from the strikers or midfielders and you can play forwards, you have to play forwards.
"But we did kill the game in the second half, so I have said to my players I am happy.
"It's a very good result, we have killed the game, and only two lapses in concentration allowed them to score.
"We could have scored two or three goals in the second half, but we didn't do that.
"That is not so important when you are ahead by three goals."
Despite being disappointed with his players' inability to extend their lead further, Van Gaal was able to reflect positively on a dominant first-half display.
"I was very pleased at half-time because it is not so easy when you see that Leicester is defending with two rows of four players and goalkeeper in their own half, and also that one of the strikers is dropping in," he added.
"Then you have a very small space to play football.
"Of course I want to play with high ball speed, but that is not always possible. Then you also need patience."
