The Heirs being evil or whatever is largely mitigated by the fact the monarchy only really has any power on paper, and use of said powers would cause civil unrest (and essentially result in the prompt abolishment of the monarchy). It is technically possible that a corrupt King or Queen could use their Sovereignty to upheave government law, but they would have to be about 50 or so years old by then anyway, so chances are less likely that they act rashly and impulsively. It is possible, but you could say the same about any system.
The key thing I think people get stuck on is the divide of power between the monarchy and the government. For example, a Government cannot be formed without the reigning monarch having a conversation with the Prime Minister to be and cordially asking them to form a Government. While said monarchy technically has the right to refuse this request, to do so would either be seen as either ignoring the will of her people, or subduing one of the major political powers in the UK - neither of which are very desirable for the monarchy that is already under pressure.
It gets a bit more complex though when you co side what has happened this week. You may or may not know that the UK has a new Prime Minister. The thing is, she hasn't been elected - the leader of the Conservative Party, Boris Johnson, stepped down, and because the Government is a Conservative Party government, it means that the Conservative party then decides who its new leader is, and that person becomes the Prime Minister as a result. Generally what happens after this is that the new Prine Minister holds a new election, this would be so that she can win the democratic election which in turn would give her greater leverage when it comes to forming policies and staples of her government. Our new one has claimed that she won't hold one until 2024 though, but we'll just have to see.
A chaotic week for Britain, to be sure.