Angelus was a construct created by an Asuran biomass known as the Hybrid. After the Hybrid escaped an attempted destruction by the Asurans, it created Angelus to infiltrate Atlantis, using Doctor Elizabeth Weir's memories to mold him into what it needed.
Angelus had no idea of his true nature and believed he was an Ancient who had been overseeing an experiment to use nanites to enhance the intellect of a planet's population, only to remain in stasis for several centuries after an unexpected culling disrupted the experiment. Having awoken from stasis, he attempted to help his 'children' develop a weapon against the Wraith, but his planet was destroyed by the Asurans. Although Angelus helped with the creation of a "weapon" that he believed could be used against the Replicators, his true nature was eventually exposed when his ship was revealed to actually be incapable of hyperspace travel on the scale that he would have required. Unaware of his true purpose, Angelus was absorbed by the Hybrid as it began to expand and infect the city. Despite being absorbed, Angelus' consciousness survived and after Sheppard stunned the Hybrid by ramming it with a Puddle Jumper while it was trying to climb the main tower, Angelus managed to gain some control over its basic functions. He created a sort of body for himself and saved Sheppard by cushoning his fall.
Angelus was horrified by the truth of what he was and knew the Hybrid had to be killed, so he teamed up with Sheppard to do it. After gaining Sheppard's trust, Angelus took over two litres of Sheppard's blood and used that and Sheppard's Ancient Technology Activation gene to manufacture a poison to kill the Hybrid while it was stunned, exploiting the fact that its artificial immune system could only attack while it was conscious, and it could thus be killed by introducing an unknown foreign antibody into its system. Angelus helped Sheppard escape, but lost control at the last moment as the Hybrid woke up. However, Sheppard was close enough to the edge of the Hybrid that he escaped on his own. The Hybrid died of the poison and Angelus died with it. (SGA: "Angelus")