What extremely bad decision do you feel the protagonist/protagonists should have avoided that would have avoided/made the entire plot super pointless?
Since it's fresh in mind, for me it's in Genealogy, with the runner up being Awakening.
Sigurd is told that Deirdre is cursed, like if she marries doom is to befall the world (I forget the exact words but you get the gist of it). And amidst a world where curses are a thing, where dragons exist, and where people hold holy and demon weapons in the names of magical crusaders, he proceeds to call the Prophecy hogwash, and chases after her anyway. Don't get me wrong, I still love Genealogy (probably my 2nd, or 3rd favorite) but I have to say that I was astounded at how badly he took the advice and said "Yo, I don't care, the power of boners prevails over all!"
Runner Up: Chrom taking Robin to the Dragon's Table for no apparent reason, instead of taking the advice that Robin and Frederick gave him being "I don't think I can hold back, or I don't think it's a good idea."
So now go ham, lay it on the protags/characters.