In Dekker's defence the planned original ending was much better than the Pred Killer. But to touch on what he also said, he had 3 years to remedy it and come up with something better once Arnie said no to that.
Now, I appreciate he's not entirely to blame, but the script was never the strongest to begin with, was it? I distinctly remember, whilst reading the leaked script, thinking to myself: "This might actually be good if it wasn't a Predator film" and that it felt like he/they happened upon a good idea and then reworked it to include the Pred.
As for calling out the fanbase, he needs to get a thicker skin. He's in a position if great privilege which I'd be willing to bet pays quite handsomely - in more ways than one! And he ought to know that, no matter how good or bad any material is, it's always going to have people who love it and people who loathe it.
Furthermore, there are ALWAYS going to be dickheads in every walk of life, and whilst I'm certainly not condoning any of the stick he's getting, it's not like it's just a couple of fanboys who're hating on this film, is it?
It's reception has been, unanimously, tepid at best. So I'd advise he/they take a look at where the movie falls flat first before addressing the idiot section of the fanbase and calling them out on their idiocy.