He does it because he has unfinished business, he’s let himself down, he did the wrong thing with the child Davros. He abandons the child Davros to his fate which is not the right thing to do, so he’s haunted by his moral failure.The opportunity to square that circle, to resolve that failure, to make up for it, to make peace with it is why he goes to Davros. He owes him, and knows in essence that he’s responsible for the creation of Davros.He’s played a part in that so he accepts that he has the responsibility to try and resolve it, and that’s why he goes.
