I think Pettersson has all the qualities to be a great captain. In fact, on a positive note, we're really lucky to have three forwards who are all prime captain material.
However, my one major hesitation with giving Pettersson captaincy when Horvat is available, especially at this juncture, is that I'm just not sure how tight he really is with the team this early. He has the right attitude and maturity for himself, but is he the one holding everyone else accountable, or is Horvat? I'm guessing the latter. The team is clearly in awe of his skill and respect him on that level, but I don't think that alone is enough evidence. On top of that, he's only been here half a year and we've already had two incidents where he was targeted and the team's response was a complete joke-- they did not have his back at all. The optics of that would be multiplied ten-fold if he was also the captain and that happened.
Also, it makes no sense to hesitate about giving Horvat the captain role because he's heavily relied on to play in all situations. It doesn't matter that your captain isn't technically your best player, it only matters that he's one of the most engaged, relied upon, and heavily influential to the team. Which he is, as much as anyone. The team seems to fall apart just as much when Horvat is missing.