I suspect the difference between a UK Home Secretary and many of the other roles involved in policing, security and the legal system is that the HS is probably only getting involved in the most serious cases, where there are already sufficient grounds for thinking someone is a very bad person to want to take things to a level that requires such high-level approval. If you're a lawyer who practices in court or a senior police officer overseeing a whole area, you see the bad people but you also see people getting found not guilty or people being arrested on suspicion but then released when the evidence points elsewhere. I claim no special knowledge of whatever sensitive matters the HS sees on a daily basis, but I'm guessing with the filtering that must surely happen before any particular individual comes to the Home Secretary's attention, they see a much higher ratio of actually very bad people to suspected very bad people than most.

Just to be clear, I'm not defending May's policies as either HS or PM here. I think she demonstrates both a lack of technical knowledge and a biased perspective that make her very dangerous to the public good, and as such I believe she is not the right person to serve as PM. I just think that those flaws are somewhat understandable and probably something that would affect any human being who had served in her former post for as long as she did, unless perhaps they came from a background particularly relevant to these kinds of issues. The real problem may be expecting any single individual to do what is currently required of a Home Secretary for an extended period, certainly without some sort of mandatory recovery period after they leave the post before they take any other high-level position within the government.