Leakers, in my experience, often have (what they consider to be) good intentions. The problem is that those intentions are, at best, misinformed. At worst, they work entirely against whatever it is they are trying to achieve/ the person they are trying to help. Or at the very least, in the process of helping their preferred person or organisation, they destroy other third parties. This is often because they are genuinely clueless - they don't truly understand what's happening with their preferred person/organisation on the one hand and on the other hand, they have no understanding of the consequences of their actions. Often mid level employees - they have enough information to be dangerous but not enough to see the bigger picture. I strongly suspect most BP/KP leakers are in this category.
[Having said that, I stand by my assertions that leadership from the top/good management can plug the vast bulk of such leaks by a) making it clear they're not acceptable and b) doing better internal comms so that the big picture is more likely to be understood by a wider range of people].
Thomas Markle might not be a leaker, but he's a classic example of someone who thinks going to the press will result in a specific outcome but whose cluelessness and naïveté just makes everything worse.