Doctor Who: 10 Times The Doctor Was The Bad Guy
1. The Time Lord Victorious
In the aftermath of the Time War, the Doctor was faced with the knowledge that he had seemingly wiped out his own people. This guilt threatened to ruin him, though his companions served to soften his hearts over the years that followed. The loss of Donna Noble undid much of this work, forcing the Doctor to face the fact that he was destined to be alone.
Travelling alone is not something that does the Doctor any favours. He begins to grow more secure in his abilities, which is fine. Yet with no oversight, or second opinion, he gradually starts taking more and more risks. This culminates in his actions around Bowie Base One on Mars. Despite the events of the base's last day being a fixed point in time, he changes events and saves three lives.
The saving is not so much the issue here, rather it is the thought behind it. The Doctor has completely given over to having full control over time, free from the rules that the Time Lords had imposed. They had been set for good reason though, as Adelaide Brooks proves with her disdain for his behaviour and her later suicide to set time right again.
The Doctor walked a very fine line here, stepping too far over to one side. Were it not for the selfless action of Brooks, he could have been another Master in the making.