Sure, you could cut hinge mortises with a router and a template, then finish ’em off with a plain ‘ole chisel. But unless you’re a practiced woodworker, the results can look, well, less than perfect. What you really need is a corner chisel. Just place the corner chisel in the corner of the mortise, strike it with a hammer, and clean out the waste with a standard chisel — perfectly square corners every time.
And with Amazon selling the Grizzly corner chisel for $6, there is no excuse for sloppy looking mortises.