Ryousuke Sugano

Ryousuke Sugano was a pivotal antagonist in Oshi no Ko. He was an obsessive fan of Ai Hoshino, whom he murdered after learning that she had children, before committing suicide to avoid arrest. Prior to this, he also killed Gorou Amamiya when the doctor interfered in his attempt to kill Ai before she gave birth.

A shut-in, Ryosuke was a mentally unstable person who turned toward idols to feel loved in his life. His room being filled with Ai Hoshino's poster clearly shows his strong obsession on her. As a result, when he learned that his top idol had "deceived" him by having twin children, he snapped and tried to kill her on two separate occasions, succeeding on the second. However, when he was proven wrong about Ai not even remembering his name, regret may have instantly hit him, which he could not properly process, thus running away from the scene and committed suicide eventually.

(Source: Oshi no Ko Wiki)