Personally I don't think Theon will be the one to kill Ramsay, that's too convenient a plot line in terms of GRRM in my opinion. I do think however he will have some agency in the retribution against Ramsay (this is of course we all assume correctly that Stannis is going to beat the Boltons
Theons doom is sealed I think, though I can definitely seeing him being one of the arbiters of Ramsays doom in respect to being the information source that Stannis requires to show the north-men the duplicity of the Boltons in regards to the Starks & being an 'insider' eyewitness to the rein of terror that Ramsay has wrought. This could possibly be at Theons trial which I think is a given, or as witnessed by the assembled Lords of the North following the capture of Ramsay and being brought face to face with Stannnis.
Whatever happens I'm sure there will be some suprises along the way.