Right, but your friends are looking at the situation in hindsight as audience members watching a movie. I was talking about Quicksilver stopping Magneto because the reason everything goes to hell in Paris is because Magneto decides to kill Mystique.
If that never happened and they just got her and left, the question would probably have never entered their mind.
Its just a line of thinking that I believe looks at characters as plot devices rather than as actual characters.
The movie established that Quicksilver is not exactly the most morally sound character, with all of the stolen stuff he has which is why he's easily convinced to help break into the pentagon.
Charles is clearly apprehensive of him, but they don't really have any choice if they want to get Magneto out.
Once they've used him for that one purpose that they needed him for, why keep him around when it could potentially muddy the waters.
I really don't think there is a need for any additional justification in the movie.
Yes, this is another reason. Even if Quicksilver were to stop Mystique, they still need to convince her to stop her crusade against Trask. The situation isn't as simple as people make it out to be.
He had just made the trip back in time, he wasn't exactly thinking in the most logical sense at the moment when immediately confronted with a potential conflict.