고 떠나다

leave for good, having done that

떠나다 is departing for good — leaving a place, a person, a life — and carries more weight than 가다. The -고 clause is what you did on the way out: 보현을 두고 떠나는 "walking away and leaving Bo Hyun behind", 자기를 배신하고 떠난 아내 "the wife who betrayed him and left". 떠나보내다 is letting something or someone go: 안 좋은 거는 빨리 분리하고 떠나보내는 게 좋습니다, "better to cut bad things loose and let them go".