Why Garak joined Tain in DS9: "Improbable Cause"

This is a comment reply under "It Doesn't matter I'm Back."

Garak did not play him. Tain implicitly threatened to kill Garak all the time in that scene — these were never just charming words, and this was never an empty threat, as all their previous associates met their maker a bit too soon — and so, Garak was pleading for his life ("I never betrayed you!") by explaining why he was there in the first place.

Tain believed him, and offered Garak a position by his side.

See: after Tain exposed his entire diabolical plan of genocide to Garak and Odo, Garak realised, that he's expendable.

Considering their common past, Garak must have understood, that Tain would never let someone go 'just like that'.

To confirm his fears, he asked, if Tain was concerned, if Garak, on potentially leaving 'just like that', might warn the others ahead of time (who would then attempt thwarting Tain's crusade), and Tain confirmed that he was not, by offering a denial by saying, that it's too late.

Odo deduced just as much, who correctly warned Garak.

Taking that all in, Garak decided to play along to join Tain.

Because the alternative would have been for Tain to order Garak's demise yet again, without the Flaxian perfume this time.