General rule of thumb is that any game with important plot elements hidden behind paid DLC is trash and when the vanilla ending doesn't even make sense as a result you're playing a truly awful piece of shit. I'm not going to bother spoilering this because this game is a truly miserable experience and no one should ever subject themselves to it.
In Dead Island you play as some stupid asshole who spends a large part of the game running errands for a guy called Rider you "talk" to over the radio (except you're a silent protag) who offers to help you get off the sewer-infested island in exchange for a "cure" for his zombiefied wife. Eventually you retrieve the "cure" and head to the final area and location where Rider is, the prison. You meet some dickhead prisoner who helps you through a few areas and eventually reach an elevator at which point for absolutely no discernible reason Rider betrays you and puts you to sleep with knockout gas before stealing the "cure" that you were heading to give to him. You continue on and make it up to the roof, take down all of his guards and have a final confrontation with Rider wherein your character(s) all magically lose their weapons at the start of a cutscene and he injects his wife with the "cure" which of course doesn't work and instead gives her exceptional individual zombie strength so she breaks out of her restraints, kills him and then becomes the final boss. After taking her down your character(s) along with the one dickhead prisoner get in his helicopter and fly away.
Shortly after the game's release Rider got his own stand-alone DLC pack which is possibly the worst-reviewed one of all time (for the record I never actually played it, I just read this stuff on the Wiki) wherein it's revealed that the dickhead prisoner was actually the true villain manipulating things the entire time and Rider was just an innocent victim in all of it who wanted to save his wife. And they charged like 10 bucks for it.
Fortunately DI2 is stuck in development hell so hopefully we won't ever have to put up with another one of the fucking things.