It seems to be a common misconception that lowering an object's orbit is much easier than raising it. I guess because it feels like it is just floating up there and gravity can do most of the work to pull it down if you give it a little nudge. In reality the energy required to lower the orbit is about the same as the energy required to raise it. In both cases you need to change the orbital velocity.
You keep using that word. As another response said, you seem to underestimate how much energy is needed to deorbit ISS. It has a mass of 450,000 kg and is moving at nearly 8km per second. There is no 'just' here.
