How deadly are they? They almost eradicated the mutant race, killing millions in one afternoon's work by destroying the island of Genosha. There's also a much-visited future where the Sentinels have taken over the world and killed the X-Men and many other superheroes.
Who's responsible? Humanity's intolerance and fear. Oh, and Dr. Bolivar Trask, a man who saw mutants as a threat to humanity and decided to build a collection of giant robots dedicated to genocide just in case. Subsequent models have come from the US government, the Norse God Loki and the Sentinels themselves, amongst many others who wanted to get involved in the killing game.
Last seen... as nano-Sentinels escaping after a killing spree during 2007's "Messiah Complex" storyline in the X-Men comics.