How Can I Butcher Animals en Masse?

This is now possible in vanilla with a feature introduced in the 1.8 snapshots. You can now /kill animals and mobs using the @e selector. For instance to kill all the pigs in that pen, you would use

/kill @e[type=Pig,distance=..10]

And similarly, you'd use Cow instead of Pig to kill all the cows in the pen. I use the radius specifier so that you don't kill all the pigs or cows that are currently loaded in your world, just the ones that are within your vicinity (10m). Note that there's only one space, between /kill and @e, in this command. Adding additional spaces will cause the command to not parse properly, and will not only kill you, but possibly every other entity that's loaded in the world.

Depending on the mods installed, /butcher [radius] should work. Of course, you'd need access to the commands to do so.

No-mod solution:

Place some water, and TNT in the water (make sure you aren't replacing the water), and light it. TNT in water will do no damage to blocks (leaving your jail intact), but full damage to entities (the unwanted animals). Do not place more than two TNT at a time, as explosions might blast other activated TNT out of the water, allowing it to do damage to blocks.

Note that the TNT will also damage any minecarts, boats, or loose resources in the area, whether or not there is a clear line-of-sight to them.