In a parallel universe, the Doctor was a Time Lord that believed that the ends justified the means, willing to kill people for mistakes they had made or dangerous experiments that they had been involved in even if they regretted their actions and showed no sign that they would repeat their errors. (AUDIO: Full Fathom Five)
Having been cut off from the TARDIS when the facility where he had landed, the DEEP, was lost and presumed destroyed, the Doctor remained on Earth acting as a surrogate father to Ruth Mills, the daughter of Eric Vollmer, who was lost when the DEEP was destroyed. When the DEEP was rediscovered, the Doctor attempted to recover his TARDIS.
However, when Ruth followed and learned that the Doctor had killed her father on the chance that he might continue the dangerous work being carried out there, she simply stood by as the DEEP's sole survivor broke the Doctor's neck and caused him to regenerate into a new incarnation whom she quickly shot dead. Ruth then waited for him to regenerate again. She was willing to do so as many times as was necessary until the Doctor was finally dead. (AUDIO: Full Fathom Five)