Being so powerful, Sidious probably knew he didn't stand much chance in a fair fight for many many years, and so instead of waiting to grow his power and his master to become weak he obviously just took the cowards way out and killed him in his sleep.
Which is stupid really, because plagueis probably had far more to teach him. But ah well, what can you do?
amazon.com/Star-Wars-Plagueis-James-Luceno/dp/034551128X <----Upcoming novel