If you need to understand why this is bad, then you need to read the dissent. The dissent makes it very clear why this decision is a bad one and what presidents can do while still being considered immune.

"Looking beyond the fate of this particular prosecution, the long-term consequences of today’s decision are stark. The Court effectively creates a law-free zone around the President, upsetting the status quo that has existed since the Founding. This new official-acts immunity now “lies about like a loaded weapon” for any President that wishes to place his own interests, his own political survival, or his own financial gain, above the interests of the Nation. Korematsu v. United States, 323 U. S. 214, 246 (1944) (Jackson, J., dissenting). The President of the United States is the most powerful person in the country, and possibly the world. When he uses his official powers in any way, under the majority’s reasoning, he now will be insulated from criminal prosecution. Orders the Navy’s Seal Team 6 to assassinate a political rival? Immune. Organizes a military coup to hold onto power? Immune. Takes a bribe in exchange for a pardon? Immune. Immune, immune, immune."