Whoa there, chief. Yes, it's true that Indira Gandhi was killed by her Sikh bodyguards, but it was a surprise to about one person in all of India - Indira Gandhi.
The problem was that at the time (1983-84), there was still a relatively credible Sikh separatist movement in India, albeit one that was losing a great deal of steam after setbacks at the hands of the Indian Army. The Sikh separatists were very much a minority, although I don't think that it's unfair to say that many mainstream Sikhs were sympathetic. Anyway, one of their main leaders had holed up in the Golden Temple of Amritsar with about 400 or so of his supporters.
Now, you must understand, the Golden Temple is the holiest of holies for the Sikhs - it's the Wailing Wall, the Vatican, and Mecca all rolled into one. Here it is:
So of course, in an incredibly foolish move, Gandhi ordered the Indian Army to take the Temple and apprehend (or better yet, kill) the Sikhs holed up within. And even worse than the fact that the Indian Army set foot within the Temple was the fact that the Temple was significantly damaged during the assault.
You must understand the rage of the Sikhs - even moderate, mainstream Sikhs who had not supported the separatists were outraged. Imagine how Jews would feel were part of the Wailing Wall pulled down by the Palestinians, or how Catholics would feel if Muslim terrorists blew up St. Peter's Basilica in Rome, and that's about how the vast majority of Sikhs looked at it.
Just about everyone on the whole Indian subcontinent saw it coming, except for Indira, in her incredible arrogance. Her own hubris killed her as much as anything else.
And that's the whole story on the Sikh killing of Indira Gandhi...