Well, I had posted something longer but the joys of crashes..anyhow.
I should remind everyone that natives do not hate and persecute magic users so much that they fear them. Barovians, guards included as they are just peasants with a halberd, fear magic, the night, werewolves, vampires, calibans, the unnatural, outlanders, Strahd, whatever. They are a people who have learnt to live in fear all their life. They wouldn't try subduing a magic user or a vampire, they likely would pretend it doesn't exist as they do instead of confronting it and if they were nose to nose with one, they would cower and run, not try to beat it.
Also, as commonly used as they are here for reasons of practicality, witch burnings and other religious burnings are an oddity in Barovia. Barovians are a supersticious lot, not a religious or fanatical one. What happens when they do confront "witches" is that a "brave" Barovian will pull out his pitchfork and gather a mob and with said mob, they will drag the witch out of her house and on the street where she will either be drowned (in villages near water sources usually) or lynched to death. If the witch were to cast magics or anything, they likely would run in fear. Inquisitors, witch-hunters, demon slayers, whatever are mistrusted by Barovians who see them as religious or institutional fanatics and they don't like that. And if said people are succesful, Barovians will view them with even more suspicion as they think that anyone who can beat the unnatural is likely unnatural himself. They usually find themself unwelcomed and forced to live on the fringe of society or just driven out of Barovia (Cerberus has been roleplaying that with Noigrim).
This is Barovia, not Falkovnia. Magic is not illegal here, neither is being of another race. Guards wouldn't go out of their way to persecute the unnatural creatures because they are just like the peasants, they are generally a fearful lot, a fearful xenophobic lot. A guard would likely turn a blind eye though, if peasants were commiting murder on a witch, because they are just like them. They might join in, but most likely would just ignore it and walk on.
What I would do with Marcus to do it is tip peasants off about a witch, then rile them up enough to have one of them pick up his pitchfork or a torch and then a mob would form to go get the witch, but if said witch would threaten them or anything, they likely would flee.
As for a native's attitude towards magic. We debated that on the rebel boards about adaptation. Since our rebels were put in direct contact with outlanders in and around the outskirts (as they would work around there), they would see a lot more than the general barovians. We realized that in the end, they would likely start to open up more and tolerate more the outlanders and their odd customs while not really liking them. I might quote some of those posts later as they were very informational (our discussions with Ferf and Jay were awesome on the topics of setting and all and really clarified a lot).