What defines an evil wizard from a regular wizard?

Me and a witch friend were talking about this, and I'm trying to figure out...

What makes a regular wizard different from an 'evil wizard' to the commoners most of us are evil wizards because we're too different from them with our magics, and maybe just a little trigger happy with the spells.

You turn an irritable peasant into a frog? Evil wizard! You defend yourself from a mob with a fireball? Evil Wizard!

We're called evil wizards over the smallest slights! but there's the actual evil wizards like necromancers and the conquerors. So how are we meant to define a regular wizard from an evil wizard under these conditions?

Maybe it's better to just ignore the commoners actually...