If Theon hadn’t betrayed Robb and sided with Balon, Rickon never would have been driven away from Winterfell to be murdered by Ramsay Bolton at the beginning of the Battle of the Bastards.
The whole North would likely be in a better state if Theon hadn’t taken Winterfell.
Jon Snow will probably have a few things to say to Theon.
Even Robb might not have been killed if Theon hadn’t abandoned him.