If episodes like "Hard Time" and "In the Pale Moonlight" and so on showcase Deep Space Nine at its best, then there is one moment in "In Purgatory's Shadow" that encapsulates the worst parts of the entire series.
spoiler
We learn that Julian has been a prisoner of The Dominion for several months, and that has been replaced by a Founder back on DS9.
And suddenly -- back on DS9 -- we see Alexander Siddig looking EVIL (tm).
He's never looked evil before. But from that point on he looks EVIL (tm) in everything he does. As if we -- the audience -- need reminding that he is a BAD GUY (tm). As if we aren't smart enough to realise that he is a bad guy just because we've been told once. We have to be constantly reminded.
It is truly appalling writing and acting -- just really, really bad. Especially when compared to things like the "Molly" scene in Hard Time and the whole of "Moonlight"