The sentence above requires a "connector" in order to connect <em>two independent clauses</em>.
First clause: <em>Douglass believed that education was important</em>.
Second clause: <em>He taught other slaves to read the New Testament at a weekly church service.</em>
A connector in the form of "conjunctive adverb"<em></em><em>(thus/therefore)</em> was used because the clauses show a <em>transitional expression. </em>It meant that Douglass wanted people to become educated and to value education. This is the reason why he taught the slaves to read.
He just wanted a quick way to get out of a situation so he blinds him because he feared for his life and didn't really want to kill he would rather escape.