That assumes the bandits acted of their own agenda, and not instigated by someone who wanted the village wiped out for whatever reason.
There's no point considering that (in the context of my response); if that were the case, Ren killing the bandits isn't going to prevent any negative consequences in the future. If anything, keeping the bandits alive means they have an opportunity to provide information on such a nefarious plot in order to barter for their lives.
Aint he a reincarnator? Probably he knows magic in fiction and he just needs to visualize it to work just like how he did with physical enhancement as well as his wooden armor
Well, clearly his reincarnation has done something regarding his talent and learning rate of magic but I would have expected he still needed time to practice. It seems instead that he's basically using these magic on the spot.
Even if he is aware of magic in fiction, that doesn't mean that the world he's reincarnated in is one with magic. Since there's only one wizard in the village, how would he even learn about it? He hasn't mentioned being granted any special abilities by a divine being and the level of education in the village seems quite low.
He also has no experience with magic (as he was from a world lacking magic) so how would he even know how to use it? When he was having flashbacks regarding using magic, he basically looked the same as he does now, which suggests he's only learned magic recently. He must have asked Dubble for information on how to use magic, and then started practicing and discovered his absurd talent and learning rate. He's only 5 and somehow become more skillful than Dubble in several different elements.
Dubble does not chant, he just generates fire out of thin air and manipulates it at will. So it seems to me that visualisation is the norm in this world and there's no advantage to be had there. There's also no modern knowledge that would explain what magic can do; I mean, how is he making branches grow and moving the tree? Plant cells just don't do that (there's only movement through growth and cell destruction). If anything, modern knowledge would get in the way of visualising that plant armour.
So yeah, he's very special somehow.