I just want to clarify the actual rule from the Supreme Court. This is straight from the rule.

Under our constitutional structure of separated powers, the nature of Presidential power entitles a former President to absolute immunity from criminal prosecution for actions within his conclusive and preclusive constitutional authority. And he is entitled to at least presumptive immunity from prosecution for all his official acts. There is no immunity for unofficial acts.

So, the president has no immunity from unofficial acts, and he has no immunity for things outside his constitutional authority. So, the idea Joe Biden can send a hit squad to kill Trump is nonsensical. No, he cannot do that.