If, when misting, you left droplets on the leaves that may have acted as a magnifying glass and burned it up. Not sure, but the picture really looks bad.

I would think about everything you did during the process and really try to figure out what it may have been.

I do not remember any discussion on soils, nutes, etc... Just lights, ventilation and water. So I would start there. Figuring out what happened is far more important than saving those seedlings.

Also, based upon my readings, it may have simply been a poor seed. Apparently, bag seeds are predisposed to problems.

Here is another sight that breaks down the growing process in a more straight forward manner that may be of some help.
The Cannabis Cultivation GrowFAQ - The Internet Hub for Marijuana Cultivation - powered by phpMyFAQ 1.6.5

Good luck!