They probably knew his anti-imperial motives pretty well, his tendency to violence, his desire to rid the Middle East of first-world influences. Just like they knew Saddam was a thug when the USA armed him to fight Iran.
At some point Muslims probably realized the US was helping them play "let's you and him fight." Then again, some seem to have no qualms about killing their religious fellows (Muslims killing Muslims.)
And along with Saddam, helping/using them to thwart the Soviet expansion was deemed the most prudent course at the time.