The Saudi regime is the biggest sponsor of Islamic terrorism in the history of the world. They, along with the corrupt CIA, basically created Al Qaeda. The US government's own documents prove that Saudi intelligence was involved in the planning of 9/11 to one degree or another. The murderous Islamist "rebels" in Syria are a proxy of the CIA and the Saudi regime. Most of the terrorist cells in the Sunni areas of Iraq were commanded by Saudi nationals. Everyday I was in Iraq, a Saudi national was trying his best to kill me and the other marines on patrol. If there is one government on earth that is a true enemy to America, it's the Saudi regime.
Saudi influence also keeps oil prices artificially high. But the number one reason to dislike them is their constant and deep ties to international terrorist organizations like Al Qaeda and ISIS....but they also are useful to us in certain geopolitical interests. Their influence in OPEC helps to maintain the dollar's dominance in the oil trade, for example. Ideally, we'd use this influence alone to maintain the petrodollar rather than getting involved in conflicts in the region.