I don't appear to possess this thought process.
People don't vote for people they think will bring mass carnage. If people had known what Blair would do they wouldn't but he still retains a 50/50 favourable/unfavourable image in the UK. We don't by any means really hate him; he's not tarnished like Thatcher or Cameron.
I do believe that a lot of politicians act differently than they promised once they get into office, which is part of the reason I said that I don't assume that all voters for a specific candidate or policy have the same intentions or reasons.
That said, if Blair really has a more favorable image in the UK than Cameron after all that the former has done, I have to wonder exactly what moral and political calculus underpins such a perception. I would find it an unfortunate situation at best that Thatcher and Cameron, for all of their faults, would be seen more negatively than someone who should stand before a court for war crimes.
I don't believe in blaming the voters for the actions of the one for whom they voted, otherwise no-one should really ever vote. More often than not, all options are bad for multiple reasons.
I don't believe in that for all voters; only ones who could reasonably be said to have been able to foresee the results of the vote and who had less harmful alternatives but didn't choose them—and then there are those who specifically vote for candidates in order to have them enact laws or policies that indeed cause harm to others (e.g., people who voted for Trump because they wanted a crackdown on the "LGBT agenda," elective abortion, etc.).
Of course, there are so many ways to weigh different issues, sometimes conflicting ones, that deciding which vote is more or less harmful can itself be a major judgment call and reliant on which issue or issues one decides to prioritize. Hypothetically, a leader could be good for the economy but awful for civil liberties, and another could be bad for the former but great for the latter. Could one really blame a poor person for choosing food and shelter over civil liberty? Conversely, could one really blame a journalist for choosing civil liberty at the cost of a state's economic well-being? Who gets to decide which vote is more (or less) harmful, and which or whose interests should take precedence?
But moreso, we cannot predict what they will actually do, what will happen 2 years from now. Things like COVID were not predicable, so we couldn't possibly have voted for the Tories based on a pandemic. Random factors are for more meaningful than concentrated votes, imo.
I think some aspects of politicians' actions are usually foreseeable, and some are not. For example, it should now be highly foreseeable that Trump would try to eviscerate the rule of law and democratic checks and balances should he win the 2024 election. It should also have been foreseeable that a vote for Jeremy Corbyn's Labour would have resulted in higher taxes for many people.
On the other hand, I believe that the January 6 attack on the Capitol could not have reasonably been foreseen by the average Trump voter in 2016. It's also often the case that the more favorably people view a candidate or party, the less likely they are to be suspicious of their intentions. A Democrat or especially pessimistic Republican might have expected Trump to try to overturn the 2020 election in case of a loss, and a Republican or pessimistic Democrat might have expected the worst aspects of Biden's presidency, but it makes sense that not all voters will have such a suspicious perspective.