I said they were justifed in being scared.
Give a Jew a gun, and if he's smart he'll put somewhere safely away, and if he's lucky he'll never have to use it.
Many mutants don't have that luxury; while there are plenty with relatively harmless powers, there are plenty more with destructive ones - and in some cases, those powers don't have an off switch. Cyclops is one of the most obvious examples here, but really, take your pick from any number of Marvel's Merry Mutants that have little to no control of their powers.
And even if they do learn control, would you feel safe around them, knowing that they could kill you and possibly the surrounding countryside if their control slips even for an instant? Would you trust them to never lose their temper? I wouldn't.