Thanks for the chapter, NGS.
There is an important factor that many here are missing. People like Arai and his gang tend to be too stupid to possess one critical cognitive ability to any significant degree, if they have it at all: second-order thinking ability.
These thugs are people who already live a lifestyle devoid of all concern about rules, regulations, and the authorities that enforce them (except in the case of avoiding the last). In the moment, they're thinking about what they want--not about what can happen to them if they try to get it or are successful; worse, they may believe as a rule that they can do as they like with impunity--because they can't conceive the possibility of punishment or retaliation. So, if anyone from Karin's group contradicts Arai's will or otherwise sets them off, they'll just retaliate as they said they will--they'll carry out their threat against Karin and her friends via Haruno. The consequences will not be equal in weight, however: it will be an irrevocable pain for Karin's group (the "many years" of "regret"; subsequent police action won't change the results), while the worst that will probably befall Arai and his people is incarceration, probably in juvenile detention given their age--and jailtime's playtime for the corrupt, in this era. Prison time won't even register as a possibility for them, and if it does, it'd probably be the aforementioned "playtime" to them instead.
Karin and her group have a lot more to lose than Arai and his do, because the latter don't have any cares.