Because Trump did this -- out of the blue and without discernable reason or cause -- MANY people, mostly Palestinians, were unnecessarily injured and died in the demonstrations and open violence that followed. People demonstrated around the world to protest this, both before and after the fact: Trump was literally kicking peace accords at the foundation just to watch them crumble, and protests didn't matter any more than history or peace did to him. He did it anyway, and no one was able to stop him despite the absolute insanity it was to begin with.
If your cousin gives two shits about Palestine at all, and this pro-Trump position isn't just performative spite on his part, tell him to read up on this issue. Trump is squarely on the side of "Christian" conservatives trying to force the second coming of Christ, it's all wrapped up in their bizarre pseudo-Christian mythology, and in the end a vote for Trump is a vote for NO PEACE, ever, anywhere that Netanyahu wants to stretch his own feet.