This is actually a pretty thought provoking question.
1) Ravan made an elaborate strategy after weighing the pros and cons.
He acknowledges that Ram has strength .
2) He being Rakshas, deceit is his first way and lawful war last.
3) He tells Marich that separated from Sita, Ram would be weak and then it would be easy for him to kill Ram.
( I think Ravan did understand that Ram would give him a tough fight , So he found an easy and a more cruel way out .He would have never imagined that Ram would bother to come all the way to Lanka building a bridge for the sake of one woman . Similarly he would have thougt Sita having no hope that Rama would find her would be coaxed into becoming his queen )
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