He stepped outside to see the stranger again, with a face like the end of time itself.
He knew he would be helping the man. After all, the stranger had tried to help his gran, and appreciated his mother. He was a man who was kind.
But not now. There was anger, simmering, and in that moment Tommy knew the stranger's girl had been taken, too. There was no way he would let that clever woman keep such a fate.
His father appeared, of course. Angry to no end, of course. That was nothing new. And for a moment, just one moment, Tommy almost turned to go back inside again. Then he saw the stranger's face again.
All of time was roaring in anger in that face. A storm that could not be stopped or even tamed. Here was fire. Here was ice. Here was so much that could not be said in the words he had at his disposal. He was an impossible man, and in that instant, Tommy knew that for all the strength his father had in his fists, for all the ire in his heart, it was the stranger that was the true danger. It was the man with the pinstriped suit and slicked back hair that should not be crossed.
He ran off with the Doctor because it was the right thing to do.
He ran off after the Doctor because any other action would have taken too much courage to perform.