As a principle, proportionally the same as any other politician in a executive post.
Of course polititians power are limited because they rely on an unstable power grounding.
But their evil actions might receive approval or liniency from powerful groups of interest at least initially.
Based on history we have examples of what excesses where comitted by superpowers. Usually it conveys the genocide, like with Carthage, The Celts, The Aztecs, The Zulus ( not to mention genocide commited by medium powers like Nazi Germany or The Ottoman Empire).
Anyway, the US empire has originalities, in one hand they didn't need to appeal to full scale genocide, in the other hand their power scope emcompasses the whole of the globe making any consequences more dramatic and irreversible.
But what your are talking here is something never seen before, that is, an evil person, with the intent to cause as much harm as possible. Something that potentially could be much worse, but at the same time is not that likely to happen for a couple of reasons:
The first most obvious I think is, to the extent in which his evil nature is known and acknowldeged, oppositon will grow and coallece against him. Hitler is the best example in the sense most people came to firmly believe he was evil incarnated.
Secondly, bad is made by polititians always in the name of good, and usually thinking they are doing good or at least avoiding something worse.
Nazis would think that bad was good, because it strengthens the specie, and nihilist might think nothing is really bad or good but just the way it is, or sth of the sort.
In any case, the risk is the president will try to do bad but by failling in that intent fall short of the worse potential or even of "the normal" expected level of bad stuff for the current circumstances.
But admitting he or she is evil, and that evil is effective, then there is no limit in theory other than his life span and the resistance they would encounter. The more evil (you can call it "intelligent") the worse the result, and even if not during his lifetime, the result might well end every life on earth.
A devil like this (this is what it is basically) would learn from all the people's manipulation and control ways history has to offer and would appeal to the most basic irrational behaviour of human culture, while mantaining some sort of justification (defending against a greater evil).
By the way, the current climate crisis makes it pretty easy to blow everything up immediatly with little effort. Some war here and there around the world or some instability to avoid effective international coordination and it could start an unstoppable chain of events that would seriously weaken human chances of survival on the planet in the short (50 years?) term.
It might be enough just to do nothing, let the forces in place play along and watch them waste the balance over the tipping point.
