As Chloe prepared to leave, for good this time, she thought. She thought about why it was so important for her to leave. From the moment she put on Fate's helmet, she knew. She knew her time with Oliver was short-lived.
She knew, long before everybody else would find out, that the Darkness they were fighting so hard to defeat had already corrupted Oliver. And as much as it killed her, she knew that she wasn't the one destined to stop it. Lois was there to keep Clark from becoming overpowered by the darkness because their love was strong enough to pull him towards the light and out of the Dark. But at much as Chloe didn't want to believe it, her and Oliver's love wasn't as pure. It wasn't as strong. They didn't have the true love bond that she thought they did. Their love for each other was dangerous. Toxic.
It was his love for her that corrupted him in the first place. His love for her, combined with his willingness to do whatever it took to save her, is what eventually led to his corruption. She knew the moment the Darkness had corrupted him. She could se it in his eyes. And it killed her inside. So she left.
She knew that leaving would only pull him closer to the Darkness and further away from those that cared about him. He would succumb to his grief, and his rage, and he would give up. He would almost destroy his life in the process. But she knew that it was another blonde haired superhero that would pull him out of the darkness.
And not with love, or kindness, or helping hands. But with violence. While everybody would try and talk the Darkness out of him, Dinah would use violence. She would stand up to him. She would fight him. But eventually she would ease the darkness out of him.
Chloe learned all of these things when she put on Fate's helmet. Learning what his destiny with her was almost killed her. But learning what would happen if she left him almost broke her heart. She had to leave for him to find his true love. If she stayed he would eventually die giving in to the Darkness to protect her. So she left.
She knew he would cry. She knew he would mourn. She knew he would sink. But she also knew that when Dinah arrived he would eventually climb out of the Darkness. It wouldn't be Oliver Queen and Chloe Sullivan. Not Green Arrow and Watchtower. But Oliver Queen and Dinah Lance. The Green Arrow and the Black Canary. It was his destiny. It was Dinah's destiny. Theirs was the true love.
Chloe knew this after she put on Fate's helmet. And that's why she left. For good.