Different airfoils stall differently. Some from the inside out, some from the outside in. Washout lets the wing stall from the inside out (the tip is effectively at a lower AOA) so that the ailerons on the ends of the wings remain effective.

Any imbalance between lift on the wings will cause a roll. If one wing stalls before the other, or PART of the wings stalls before PART of the other wing, with or without washout, you get an abrupt roll... aka a "snap".

Also.. aileron trim. Te wing with down aileron trim effectively has flaps deployed (however small). One effect that flap have is that they LOWER the critical AOA (though they produce more lift for a given flyable AOA). Thus, if your left aileron is lower than your right, the left wing will reach stall sooner than the right (at a lower fuselage AOA), cause a snap to left when you reach stall.