Dark sector models with massless or light mediators naturally introduce elastic and inelastic self-interactions of dark matter (DM). The elastic scattering allows the gravothermal expansion and collapse of the DM halo. The halo ends into a singular state and may form a massive black hole. A mild inelastic scattering can significantly accelerate this evolution. Together they provide a potential explanation for the formation of super-massive black holes at high redshifts