Having been cut off from the TARDIS when the facility where he had landed, the DEEP, was lost and presumed destroyed and also losing the key because General Flint had taken it, 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.
Shortly after the TARDIS was lost, the Doctor arranged for himself to get hired by the Sunbelt project which, like the project Vollmer was working on, was geared to provide renewable energy for the Earth.
Knowing the technology was more advanced than anything Earth was currently capable of, the Doctor concluded aliens were involved, and he planned to contact them so they could help him recover the TARDIS.
Discovering experiments designed to transform humans into electromagnetic signals, the Doctor refused to help and instead tricked the scientists, incinerating one with sunlight, and leaves one to suffer in solitude while trapped as electromagnetic signals, unable to be restored to human form. (PROSE: Sustainable Energy)
When the DEEP was rediscovered, the Doctor attempted to recover his TARDIS. The Doctor kept trying to stop Ruth from coming with him, claiming it was too dangerous to go down into the DEEP, almost to the point of begging.
However, when Ruth followed while the Doctor angrily told her he had done his best to keep her away and he would not be held responsible for the consequences, and learned that the Doctor had killed her father on the chance that he might continue the dangerous work being carried out there. She also discovered that the Doctor planned to kill her as well when she began to learn more about the events at the DEEP. Later she simply stood by as the DEEP's sole survivor General Flint, who had been mutated after being attacked by Vollmer, broke the Doctor's neck while trying to make him swallow the TARDIS key. Ruth told Flint that the Doctor had told her he could regenerate after she'd asked him about death and reincarnation, and that he had thirteen lives but had used the majority of his lives up. The Doctor's neck being snapped 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)
Believing that the ends justified the means, the Doctor had no qualms about using people and sacrificing them. To that end, he was 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. The Doctor had a callous attitude, especially when it came to speaking about death and killing without caring about the impact of his statements. He also had no problem lying to people to achieve his aims, nor did he have problems with bribing or threatening others. He also had no trouble - moral or otherwise - getting his hands on classified information.
He openly stated that he travelled around looking for situations which had the potential to put others at risk.
Despite his stance of never carrying a gun or other weapons, the Doctor showed no hesitation in killing people with one when he had the opportunity. He would also try to make his enemies see the consequences of their actions even if they did not care. He was also unhesitant about threatening people if he had no alternative. But despite the darker sides to his personality, he was capable of making promises and keeping his word, though it was hard to be sure how much of it was for his own ends. He was capable of caring about others, and keeping promises to look after people when he could quite easily not bother with letting them know he existed.
The Doctor frequently used his "John Smith" alias in order to gain fresh information that could be of value to him, allowing him to act. He was also not above using his UNIT credentials in order to gain the trust of others at the same time. (AUDIO: Full Fathom Five)
Even though this Doctor is a unspecified Incarnation, there is a possible theory that this Doctor could be an alternate version of the 7th Incarnation of the Doctor. Both believed that the ends justified the means. Benny Summerfield once said that to the Seventh Doctor in Love and War When he asked her what she thinks of him after Ace departed. However the main difference from the two would be that the main universe 7th Doctor would never use himself to kill his enemies, but rather using others by manipulating them and using his enemies and even allies as chess pieces to justified the means. This unbound Doctor has none of those qualities and instead chooses the easy way to resolve a situation without considering the bigger picture. Ruth's comment that the Doctor "had already used most of [his regenerations]" further suggests that this Doctor has used at least half of his regenerations by this point.