Why had Mazer been dishonest to Ender and do you think ender could have handled the truth?

Mazer was dishonest to Ender in order to push him to his limits and bring out his full potential. He believed that Ender needed to be tested in an extreme way to prepare him for the challenges he would face. It's possible that Ender could have handled the truth, but Mazer felt that deception was a necessary part of the training process.