One way to hold this word: en- means put into, and the thing you are put into is slave. So the picture is a door closing behind a person. They still act, they still work, but the choosing has been taken out of their hands and they cannot walk out. Both uses of enslave stand on that one picture; they differ only in who is holding the door.
From the word’s history
The packet's Wiktionary tree gives en- (through Old French, from Latin in-, 'put into a state') plus slave, so the word is literally 'to put into slavery'.
封包裡的 Wiktionary 字源樹把這個字拆成 en-（經古法文，來自拉丁文 in-，表示「使進入某種狀態」）加上 slave。字面就是「使進入奴隸的狀態」。這個「從外面加上去」的方向感，今天兩種用法裡都還在。
Wiktionary entry supplied in packet (kaikki:3f6f69a190036ae4:1189125359) — etymology tree: Latin in- > Old French en- > English en-, plus slave.
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