I know that the Scarlet Witch was the reason behind M-day and the loss of most mutant powers but is this the reasoning behind Vision's cold response(see Avengers vs Xmen #0) to her showing up at Avengers Mansion?
This answer contains spoilers from Avengers Disassembled:
In Avengers Dissasembled the Scarlet Witch becomes dominated by the entity inside of her. She becomes insane and starts attacking the Avengers. She manipulates the Vision into crashing the Avengers Mansion with a Quinjet. He is later destroyed by She-Hulk due to these actions. The Scarlet Witch is eventually stopped and turned over to Magneto.
Not to mention the Scarlet Witch and Vision were married. The Vision was created using the brain patterns of Simon Williams (A.K.A. Wonder Man), who also had a thing with the Scarlet Witch -she even left the Vision on the Heroes Return saga for Wonder Man (who was presumed to be dead but is brought back by her)- so you know, he might also hold a grudge about that stuff too.
The Scarlet Witch did something even worse then that.
"Decimation" is a story line event published by Marvel Comics in 2005, spinning out of the events of the House of M limited series. The event started with a one-shot issue and took place in a number of various series all carrying the "Decimation" logo on the cover. The 2005 mini-series Generation M, Sentinel Squad ON*E, X-Men: Deadly Genesis**and X-Men: The 198 were all launched specifically for the "Decimation" story line. The various stories were collected in five trade paper backs.
The story line focuses on the ramifications of the Scarlet Witch stripping nearly all of the mutant population of their powers, thereby reducing a society of millions to one of scant hundreds. This event, which occurred on November 2 according to X-Men (vol. 2) #191, is known as "M-Day" in the Marvel Universe.
Where the Scarlet Witch helps Magneto and the US government "depower" nearly all of the mutants alive at the time.
There are at least "around 300" with 198 depowered by the US government and Magneto with 102 being depowered or killed by the Scarlet Witch and a 199th mutant (Mutant Zero) having disappeared "off the record". Over 198 mutants have already been identified post M-Day with at least 30 of said mutants have died since the mutant Decimation hit.