As a child, Danny's father was accidentally killed by a lizard-like alien, only for Danny to kill the creature in response even as it pleaded for mercy. Feeling that he had found his calling, Danny took great pride in killing aliens, convinced that they were all out to destroy Earth. When he joined UNIT and learned about the Doctor, he was disgusted at the news that UNIT protocol in an alien invasion was to focus on keeping the Doctor alive, convinced that the Doctor was only saving Earth for some ulterior motive.
After joining Wonderland, Danny acquired the mask of Azrael the Necrotist — artists who saw death as the only true art — using the knowledge and expertise he gained from this mask to develop a devastating weapon capable of eliminating all non-human life in its blast radius. Having acquired the TARDIS when the Eleventh Doctor was forced to auction it off to acquire financial resources to save Clara, Danny intended to use it to release his weapon onto an alien market, eliminating countless alien species regardless of whether or not they had even heard of Earth.
Although he attempted to win Annabel Lake to his side when she and the Doctor discovered him, Annabel only went along with his plan so that she could escape her current captivity, killing him with his own weapon while the Doctor regained control of the TARDIS and disarmed Danny's weapon. (COMIC: The Blood of Azrael)