Deadly Assassin was also a huge shock to fandom's system at the time because, before it, Time Lords had been imperial, distant, and mysterious. Suddenly they became realpolitik, corruptible, and able to make mistakes. A lot of fandom screamed. Other parts cheered. (The story was written by Robert Holmes, who was (almost) always an excellent writer for the series.)

Real corruption in Gallifrey came later. :)