If you can keep it in that friend area, sure, but we both know it’s just a stopgap until you find someone better to date or have sex with. An ex is an ex for a reason. Don’t get so caught up in ex-turned-friend sex that you forget to look for other partners.
Nezumi has been through hell from point one of his life. He’s had everything he’s cared about torn from him, but it never broke him. It only made him vengeful and hateful. Affecting Shion to the point where he shot someone with no remorse crushes Nezumi. He made Shion a monster. And it kills him.