I've had my fair share of crazy comments and messages on youtube too Paal. I think some people think you have nothing better to do than sit on youtube all day every day to respond to silly questions that they already know the answers to, make requests for songs that you've already post, or to tell you how evil you are for using a steel needle on a record.
Unfortunately, there's a lot of weird & impatient people out there, and youtube seems to attract them all

I've had quite a few messages sent telling me that the sound quality of my videos is too poor for their own use, and they want an MP3 of the song from me, without a please, thank you or kiss my a**.
On the opposite side of the coin, I've had some remarkably nice comments and messages sent, including several from people around the world who seem to think the sound quality is good enough for them to broadcast over radio- which they do.
I found that 99% of people who watch my videos have a genuine interest in the music I post, and really appreciate the effort of posting them (as small as that effort is), but the other 1% are either idiots or trolls who either want to complain, be smart a**es or cause trouble.
With so many videos posted, I got to the point awhile back were I had to switch off my PMs, because I'd be getting 10-20 messages every day. Most of them were decent messages, but I just haven't got 2 or 3 hours a day to sit down and answer everybody, so it was easier to just switch it off.
I also now simply delete any strange & rude comments left, and if the person becomes a problem, I just block them from leaving further comment. I have better things to do than play their game lol.
I guess in your situation, I would contact the person & appologize for not responding quicker, but explain to them that you haven't logged on in awhile, you've been busy with studies etc or whatever the case may be, and that you do appreciate their input. Any further problems with the person though, & they'd be gone
Lifes too short to put up with other peoples baggage.