Later on in both the manga and the anime, it shows Kitazawa Yuki dropping a bottle during the time he had Eiri trapped inside of the dark apartment one night. This suggests that Kitazawa was actually intoxicated when he decided to act upon his more immoral thoughts toward Eiri. Which normally, as said in the manga, Kitazawa realized Eiri was still too young. Alcohol, taking away rational thought. Two men however entered the scene and he let them, shallow price of 10 dollars, go 'first'.
In what is determined to be a fit of fear, as well as anger, panic, but most commonly assumed as defence, Eiri managed to get ahold of a gun and shoot Kitazawa. Kitazawa's life ended shortly after. Still, afterward it has left scars on Eiri that he will carry for the rest of his life.
Tohma Seguchi was the one who found Eiri at the apartment and comforted him afterward. He feels responsible for what happened to Eiri because he was the one who hired Kitazawa as his tutor, and feels that he didn't look into him enough and could have prevented the disaster. This is a common assumption as to why Tohma is often so concerned and concentrated on Eiri's health and personal life while he is grown.
On Eiri's part, it is shown that he had cared for Kitazawa, but it not clearly shown whether it was a love or just a friendship he desired. Eiri later took on Kitazawa's first name, Yuki, as his pen name.
However, in the manga, it's shown that actually Eiri had a deep affection for his tutor. Which is often a reason why most people assume Eiri was not as resistant until others were brought into the scene. Though it clearly shows in both the manga and the anime that Eiri was shocked, and possibly fearful.
Kitazawa has a younger brother called Yoshiki, who is an MTF transsexual. In Gravitation 2, it is also revealed that he has a young son named Riku.