It is pretty clear from the video that Tiax is as insane as all others cell mates. All of them have similar lines with different facets of madness. The tone is preserved through music and Irenicus narration, no inconsistence at all.
We all know how Bioware didn't have time to flesh out all NPCs in BG1.
Not an excuse. There is nothing in the main plot that required Tiax to be put into Spellhold. And yet there he is, placed by the writers on purpose.
