3. Why Couldn't Klaus Control Himself Better?
For someone so old, Klaus certainly didn't have a lot of self-control. In fact, he was worse at controlling his impulses than many much younger vampires (Katherine, for example).
Maybe it was because Klaus felt that nothing or no-one could stop him or maybe he was just too spoiled to care about learning to control his worst side. Whatever the reason, Klaus did many heinous things, not because they benefited him too much but simply because he wanted to.