Blow up the mine's air purifier
The mine's purifier
Excellent, my friend! Here. Take this explosive switch. Hook it up to the air purifier and get the hell out of there. We've arranged it so that the town's mutants will be in the mine.”— Jacob the Chemist
After the prison break, talk to Jacob, and he will offer this quest to blow up the mine's air purifier and kill the super-mutants. He gives you a detonator, which must be attached to the purifier in the mine. Once this is done, you are automatically transported out of the mine, receive 1500 xp, and every super-mutant in the town is killed. Jacob offers you a small cache of weapons as a reward when you speak to him.
Your Karma will drop fifty-points, killing off every super-mutant in the town, and dooming the survivors.
Once you receive this quest, you can instead turn the gang in to Marcus for 2500 XP.