I realised a small observation regarding the people who took up the post each year. Apparently each of the seven people, were charachteristically different from each other, and exhibit some charachteristic unwanted vices (unwanted in the sense that we wouldnt want it in ourselves) .
- Quirrell, in the first year, is some-one who is cowardly and spine-less
- Lockhart, in the second year, is some-one who is boastfull and fraudulent.
- Lupin, in the third year, is some-one who is miserable.
- Moody, in the fourth year, is some-one who is cranky, Vichitr and actually deceitfull(no more spoilers)
- Umbridge, in the fifth year, is some-one who is corrupt and un-just.
- Snape, in the sixth year, is some-one who is vengeful and un-courteous.
- Slughorn, in the seventh year, is some-one who is lazy and nepotistic.