Main breaker keeps tripping after installing light switch

This is at an old house without properly labelled breakers, so we turned off the main breaker (bad idea?) to install a combo light switch + outlet (a left-right light switch paddle on top and a single outlet below.) I hooked up the white neutral wire to the silver screw, one black wire on the brass screw on one side of the light switch and the other black wire on a brass screw on the other side (there are 2 brass screws on the right side of the switch and 1 silver and 1 brass on the left side). After hooking this up, we turned on the main breaker and it trips. Haven't been able to turn it back on since. Since this is Sunday, it will be hard to find a electrician. Did we just blow the main breaker? Turned off all the other breakers before trying to reset the main one and it still trips.

One other odd thing about the wiring: one of the black wires is NOT a terminated end. Instead, for whatever reason, it is a segment of the wire that has been stripped of it's casing and the copper exposed. This was hooked up to the previous switch which also was a light + outlet combo. It was working fine, so I didn't do anything with it.

Any advice would be much appreciated. Thanks.