Façade
He had many personalities. He could be logical and disconnected. He could be gentle and solicitous. Often he was zealous and jaunty or destitute and downtrodden. But the facet that most disturbed him was the murderer, the killer of countless innocents. He'd demolished planets and decimated populations. He had even committed the genocide of his own race.
This is why he travelled alone; he deserved loneliness. He could not bear to cause any more destruction. Still, it had been so long since then, and it was not in his nature to capitulate (especially to his own conscience). He needed a companion, a friend. To save people (make amends for his faults). It was a duty he could not deny, as the last of his kind.
So he tried. He traveled with many companions. He gave them all of time and space, but lost them in the end. Rose: stubborn and patient, gentle and strong, trapped in an alternate universe; Martha: rational and clever, mature and responsible, terrorized by his oldest friend and enemy; Donna: sassy and strong-willed, defiant and dependable, so far gone he was forced to erase her memory. Rory: so clueless yet so wise, lively yet constantly dying; and Amy: unique and proud, beautiful and brilliant - the married couple sacrificed their lives for their love. No matter how careful he was, he ended up hurting those he cared for most.
No wonder the Daleks called him "Predator", or the mere mention of his name terrified his foes. His presence meant danger, death, devastation for all. He left nothing but pain and fear in his wake.
His name had meant "healer, protector". He'd been a savior, a god. But he was the death of all good: Rassilon knew the Doctor was the death of his own.