Someone was discussing with me siyata dishmaya during the second World War. He spoke about how Churchill came to power and inevitably fought against Hitler. So he quoted Rav Miller from gateshead saying that it was lev melochim beyad hashem. If that is the case, then surely it should apply the other way around, as dark as it sounds, that these despotic horrific leaders too would be in control by hashem. Any thoughts, or meforshim would be enlightening.
The Chasom Sofer (parsahs V'Yagash) and other meforshim say that Lev melech beYad Hashem doesn't mean that a king does not have bechira at all because as this question points if that were the case no king would be accountable for his actions.
Instead it means that when davening to Hashem to change the kings mind for the good it is easier to have his mind changed that of a regular person. While for regular people Hashem rarely interferes with their bechira he does so more readily and frequently for a king. At the times that Hashem did not do that his bechira holds him accountable for the good and the bad.
However even if one were to assume that he indeed has no bechira the answer to the question of why is he held accountable isn't so different than the general question of why is anyone held accountable for harming someone else else, didn't Hashem want that person to be injured or it would not have happened ? While some like Rav Saadya Gaon say that Hashem gives people the bechira to injure others even when he doesn't want the other person harmed, those who disagree say that Hashem has many other ways of causing the second person to be injured he had it done through this person because he was anyways guilty of wanting to do so (magaliglim chova al yedey chayiv)
Similarly a guy like Hitler wasn't this great guy who all the sudden when he found himself the German leader decided to do all sorts of evil. He was always an evil person way before he became the German leader. Being that Hashem wanted these things to happen he made Hitler the German leader. As leader Hashem saw to it that he did not have a change of heart even at times when it may have been beneficial for him to do so. Ditto for Winston Churchill. Because Hashem wanted that outcome he has Winston Churchill elected. Had Chamberlin stuck around he would have done the same things as Churchill but his own actions and way he lived his personal life before getting elected caused him to lose that zchus
This week's Toras Avigdor actually mentioned this issue (I think this is a different Rav Miller whom you quoted). He says quite openly that the real blame is on the Jewish nation:
In case you're interested, in other places he says much more than this. I can find it if you want.