People are freaking out that the president can legally kill people now but that was essentially already the case, de facto. Obama did it via drone strikes, for example, Anwar al-Awlaki, who was involved with the Taliban but never given due process, and later his 16 year old son Abdulrahman al-Awlaki, who was never even accused of terrorism - both American citizens. Of course, Bush also set up a completely illegal system of detention without trial at Guantanamo Bay, which also included American citizens and which continued long after his term. There was also of course the illegal mass surveillance program that began under Bush and continued through Obama, Trump, and Biden, with the only legal action being against the person who exposed the crime.

In all of those cases, the Justice department simply chose not to investigate or press charges, as is within their power to do. If the president straight up shot someone in the middle of Fifth Avenue, it would be up to the Justice department to decide whether or not to prosecute, and if they say no, that's that (though it would also be possible for congress to act via the impeachment process, which would require a majority of the house and 2/3 of the senate to be on board).

This ruling doesn't give the president a blank check, but rather, it gives the court an easy legal argument to give the president a pass on any case they hear. The court can still rule that something wasn't an official act. Practically speaking, before they still could have still found the president innocent for whatever bullshit reason they could come up with, but they're now saying that they don't even have to pretend to have a reason.

Of course, if the president wanted to start killing Supreme Court justices or other political opponents, a piece of paper was never going to be the thing that stopped that. Whether the president can order the military to gun down congress is just a question of whether the military decides to listen to them and whether anyone manages to stop them. It was always the case that if you can kill anyone who could find you guilty, you can do whatever you want. On the other side of that, even if the ruling did authorize the president to kill all of his political opponents on some technicality, he would still face the same obstacles if he tried to do it.

What the law says only matters insofar as it can be enforced, and ultimately laws represent threats made by the powerful towards the rest of us, and among the powerful the way of settling disputes is power, with legal power being but one of many forms that can take.