Anybody know what this guy is all about?

Viekang is an NPC I meet at the pub in trademeet. He says "What... no! No! A Bhaalspawn? Here?!"

Then no matter what I say he goes " No, no! Not after all this time! I won't... ah, AHHH, NOT AGAIN!"

Then like a lighting strike hits him from nowhere, he takes a pile of damage goes unconscious and disappears. What is up with this guy? Was he in Baldur's Gate I or something?

No, not in BG1 to my recollection...but he may be back in ToB...I won't say how or when, where or why...but you haven't seen the last of him...or maybe you have.

He appears in Throne of Bhaal

Yes you meet him very early on in ToB and he explains his weird behavior. Rather strange guy.