I don't see many world leaders on that list. But that's probably all well and good as you asked for threats to humanity. The top one is probably the closest so far. I think that at the moment he is busy destroying his own country. By the time he is done with that, I expect justice will keep him engaged elsewhere, so world destruction will be put on hold.
I put Trump just to see the results but I do think he is loose canon. I'll add that any leader that doesn't recognise climate change and policies that seek security for the planet and humanity, is a threat to humanity.
#4 Actually I think there are good arguments for each of Trump, Putin and Xi. One thing Trump has in his favour is that he is likely to only be around a couple more years. I think there is a good chance that he will be forced out of office late this year or early next and if not should lose the 2020 election. Both Putin and Xi are younger guys who have seemingly crushed all internal opposition so could easily be around for 10 to 20 years. In terms of criminality Trump is an amateur compared with Putin. The main danger from Trump is the undermining of democracy and the US democratic institutions during his remaining tenure plus the possibility of some catastrophic error of judgement. On balance I see Xi, though not a criminal in the same way as Trump or Putin, as presenting the worst long term threat. I think within 10 years China under Xi may be able to use its economic power to impose the kind of controls on global society that it is currently able to impose on its own citizens.
#5 You make a compelling argument in the case of Xi, whether or not leaders of more democratic countries would allow such controls however pushes my answers towards Putin instead. As you said, Trump seems amateur in comparison, too much of a bumbling idiot in some ways.