I had the strength.

I did it." Her clear grey eyes looked into his, and his doubt began to break away before the truth.
"Make me believe it!" he cried, his voice breaking.

"Oh, God! Make me believe it before I die!" "It is true," she cried, in a low, strong voice that carried belief to his breast in spite of such reasoning as still had some power over him.
"It is true, and you shall believe it; and if you will not, the man you have killed, the man I loved and trusted, the dead man who knows the whole truth as I know it, will come back from the dead to prove it true--for I swear it upon his soul in heaven, and upon yours and mine that will not be long on earth--as I will swear it in the hour of your death and mine, since we must die!" He could not take his eyes from hers that held him, and suddenly in the pure depths he seemed to see her soul facing him without fear, and he knew that what she said was true, and his tortured heart leapt up at the good certainty.
"I believe you, my child," he said at last, and then his grey lids half closed over his eyes and he bent down to her, and put his arm round her.
But she shuddered at the touch of his right hand, and though she knew that he was a condemned man, and that she might never see him again, she could not bear to receive his parting kiss upon her forehead.
"Oh, father, why did you kill him ?" she asked, turning her head away and moving to escape from his hold.
But Mendoza did not answer.

His arm dropped by his side, and his face grew white and stony.

She was asking him to give up the King's secret, to keep which he was giving his life.