Destro locks can do as much dmg as any other class. Just a little less than a hunter.
Their burst is amazing, and if you have the skillz, you can easily beat a rogue.
Obviously, you'll die countless times. The survivability in general is terrible.
But as long as you stay in groups and know how to pick your fights, you'll be fine.
Demo is nice for mobility, but less damage.
And Affliction actually got a buff last patch, and the dmg isn't bad, but it can be outhealed quickly and you won't survive anything 1v1.