Have you? For 20 years. Constant torture, constant mocking. Have you considered an alliance just so they wouldn't destroy your people and knew they'd betray it one day anyway. Have you looked Davros in the eyes and seen only hate? I have.
I brought the Master back. My perfect warrior when my other failed me. And Rassilon, he twisted him, he turned back on itself for his own gain.
If he can do it to destroy someones mind Rose why can't we to fix it. To fix the world. Why shouldn't we?