Reason for a fried wire?
OK you don't really give us much information, but I thought I should address what we know. First I recommend to all DIYselfers to never make branch circuit changes like you did since you really do not have the experience level. If the original electrician install 14/3 you have to assume he had a reason, and we may never know why.

So I would advise you take out the 12/2 and reconnect your circuits back to 14/3 connecting color to color. If nothing else at you new receptacle addition you can wire nut it off and label it "unknown" and leave it as such in the back of the box where it can do no harm. Also I am bothered by your splice in the attic I am assuming you have installed that splice in a box and cover and not left it exposed in open air.

If this doesn't satisfy your question I suggest you take @isherwoods advice and maybe even bring in an electrician to help you trace out wires and explain what is happening in your system.