Question

why does kmno4 acts as self indicator

Answer 1

KMnO4 solutions are dark purple. When used as a titrant, as soon as the endpoint is reached and the KMnO4- is in excess, the solution has a permanent pink hue (provided that the solution is initially colorless). Thus KMnO4 acts as its own indicator.

Answer 2

KMnO4 has its own colour. It is pink in colour. When it is used for titration, at the end point, the colour of the solution changes making it a self indicator.
If the solution is colourless initially, it changes to pinkish colour at the end. Otherwise, it changes colour according to the combination of colours.