They knew perfectly well what they were getting. They chose to ignore the dark side, in the hope of the good things he promised.
Very good.
The rest of the world saw the shiny face and wanted to see nothing else until it could not ignore what was before them.
The people who saw the evil and knew it for what it was (and many of the youth were oblivious, for they had not been taught what evil was) either left, resisted, or went along out of fear.
Those who bought in from the inside likely knew exactly what they were getting, but it played to their emotions and benefit at first, or they thought it neutral,
to them. That was not a principled stand, not an absolute judgement against a metric of what is universally right or wrong, but a 'What will fill
my belly?' mentality.
Much is at risk when principle is cast aside.