I'm not an expert on neither the Scanian War or snaphaner in general. However, I am studying Danish history and I have a general understanding of the Swedish wars in the 17th century, so I will try to answer your question.

You seem to have a good understanding of who were considered snaphaner and what they did, so I won't explain that further. But it is important to point out that snaphaner was a term that covered brigands and bandits, Danish Free Companies and peasant militias. But in the context of the Scanian War it was a Swedish term for anyone who fought on the Danish side, while not being a part of the regular Danish army. Because of this, they were, as you've written, treated as outlaws.

However, in order to answer your question, we first have to establish who were snaphaner, because there is a disagreement on this question. Historians like the Swedish Ålf Åberg claims that the snaphaner primarily were people from outside the peasent population, such as vagabonds, discharged soldiers, brigands, farm hands etc. According to Åberg, the peasants of Scania were not willing to leave their farms. However, we have veryfied sources who show that a lot of peasants did in fact make up a large portion of what we would call snaphaner. However, the number of peasants dwindled as the Scanian war went on, probably because they had to ensure their farms and livelihood. Some Danish historians like Knud Fabricius claims that most snaphaner in fact were peasants. But again, we have to remember that the term was used very broadly.

Interestingly enough, we know that the snaphaner looted, plundered, robbed and murdered the local Scanian population just like the Swedes. And we now that the Free Companies recieved supplies from the Danish army in the winter, but they had to fend for themselves in the summer time. As the war gradually became a war of attrition, many peasant snaphaner accepted the Swedish offer of pardon in 1677 to lay down their arms. But we also know that the relationship with the local population and the snaphaner deteriorated from 1677 and onwards. This could indicate that the snaphaner after 1677 mainly were men who had no farm to return to or as Åberg claims, vagabonds, brigands, farm hands etc.

My guess is that the snaphaner who did most of the looting and plundering weren't local peasants. Think about it, would you likely rob and plunder the people in your neighboor village or local area? Probably not. I think that the people who did most of the looting were non-locals, soldiers of fortune, mercenaries, brigands and most importantly people who didn't have a home to worry about in Scania or people who had already lost their homes and saw no other means of providing for themselves. But this is purely guessing, because the term snaphaner, as I've mentioned, could be anyone who opposed the Swedes with force, regardless of their background.

So the answer to your question is: Yes. The snaphaner did do all the horrible things to the local population as the Swedes did, but the level of violence probably depended on what kind of snaphaner we're talking about. I hope my answer made sense to you.

Source: https://danmarkshistorien.dk/leksikon-og-kilder/vis/materiale/snaphaner/

The page is in Danish, so I don't know if you can read it. But the page is made by historians of the University of Aarhus in Denmark. If you do read Danish, then the page is excellent in providing short descriptions of certain historical themes and periods as well as providing the relevant sources.

Edit: I know that the rules of this sub forbids guessing. While I think this rule incites a positivist approach to history, which in my opinion is wrong and problematic, I have to follow these rules. So please note that I'm only guessing on who looted the most, but I'm providing veryfied sources on the snaphaner lootings themselves, and therefore answering the question according to the rules.