I've worked with someone like this, we had to bring in an expert to learn the system in preparation for a rewrite (and let him go) and he was thoroughly obstructive and unpleasant to work with. That really won't look good on his resumé...

I'm intrigued by the comment about "emotional problems". I would argue that sometimes it's worth the effort and investment and sometimes it's not; bad behavior can come from otherwise great people that are simply going through rough patches. What's really not worth investing in is someone with a bad attitude (including those who harass co-workers / generate office drama), or who lacks the intellectual capacity or curiosity to improve.