I can think of a number of reasons why monarchs may suffer somewhat less from the 'power corrupts' norm. (1) often educated from childhood to use power wisely (2) often feel their power is legit and therefore less fearful of overthrow (3) tend to get better 'press' than other autocrats so that abuse of power less noticeable (4) often have continuity and structure in their advisors inherited from previous monarch.

Despite this, there have been some pretty nasty monarchs through history - even ones that are thought of as great like Good Queen Bess. However, if I had to live in an autocratic state I would prefer an established monarchy, all others things being equal.