Inu Yashiki (いぬやしき) is a manga series written and illustrated by Hiroya Oku. Published by Kodansha, it's serialized in Evening magazine. It is published in English by Kodansha Comics USA
The series follows Inuyashiki Ichiro, an elderly, friendless man with an uncaring family. After he and a teenager are struck by a mysterious explosion in a public park, hinted to be of extraterrestrial origin, his body is replaced by an incredibly powerful, but still outwardly human, mechanical body. He quickly realizes the extent of his body's powers, and after saving a homeless man being beaten by a pack of teenagers, he decides to dedicate himself to doing good. He uses his powers to cure people with incurable diseases and fight crime. At the same time, elsewhere in the city, the teenager who was with him at the time of the explosion, Shishigami Hiro, is also exploring his powers. Unlike Inuyashiki, Shishigami is a sociopath who uses his new found abilities to murder classmates he dislikes and innocent people, including young children. Inuyashiki sees him briefly after one such killing spree, but is unable to stop him. Shishigami also tells one of his friends, Andou, about his powers and kill the classmates who had been bullying Andou in front of him. Understanding how dangerous Shishigami has become, Andou sets out to find a way to stop him. He realizes another person with comparable powers to Shishigami exists after reading about Inuyashiki's exploits in the news (although nobody knows who is responsible), and makes contact with him.