Then Biden can have him wacked and when caught, he has immunity.
Once Trump is gone, Biden will probably fall asleep from the adrenaline drop, and the VP will have to take over. Win win for freedom!
Actually, I am quite surprised I have not seen more people on the internet point out the possibly of Biden taking advantage of the lastest Supreme court ruling for him to take care of Trump, but let us be honest, even if the president have full immunity for actions taken care as their presidential duty, it would be harmful for the country if Biden decided to take the matter onto his own hands, about half of people in the country would question whether the presidency is actually held by a king in disguise, it would be very bad for the international image the USA is supposed to keep as beacon of Democracy. So I rule out Biden would do something like it.
I think that Biden holds himself to a higher standard or at least his party would hold him to a higher standard. But yes, potentially anything goes now in the political arena. Biden could, for example, consider Trump a threat to national security as he held secret documents, did not want to return these and may be perfectly well admitting people into his mansion that may "casually gain access to these". One quick CIA action anytime Trump goes abroad and all covered by immunity as is an "official act" to protect the US.
I know this will not happen, it is just an example of how absurd the ruling is.