I dunno. I sorta get the idea that HXH's world is kind of what the shady parts of our world are just...everywhere rather than in the back allies of some third world country. I think part of the problem here is that Hunters are given a lot of free reign, so it's hard to pinpoint what exactly they do. Sure you have the guys who are black list hunters and gourmet hunters and stuff, but for the most part it's the sorta thing where you can pretty much just say 'Imma go do this' and as long as you have a license, it's pretty much okay. I think we can all pretty much agree that the world of HXH is pretty...depressing? I mean it's mostly hidden by Gon and Killua's exploits and adventures, but as you pointed out, assassination is a perfectly reasonable way to earn money, magical beasts exist, guys like Hisoka and the Spiders are pretty much able to do what they want since the Hunters association doesn't openly try to stop them, and since Hunters are technically not police officers, there's technically no one to stop this from happening. Being a mook in the HXH world has to be like awful, since it's not as if every single person is a shingen ryu master. In essence, I'm saying the line between good and evil in HXH is pretty much always skewed to a point that even the most heinous of bastards looks like a saint, and the best good guy kinda comes off as a hell bent psychopath.I always find it rather ironic that people praise HXH for its depth and yet often misses its worldview completely. Whether this is because it's too subtle or that its worldview often does not make sense, the end result is that HXH is clearly not as deep as people claim it to be if people rarely get its underyling themes.
In the world of HXH if you're an aspiring evil guy, you should consider skills in computer hacking, embezzlement, selling drugs, or become a politician. Brute force is the last thing you want to rely on in the world of HXH because the Hunter's Association will strike you down with extreme prejudice, and even if you're Meryem-strong someone will just launch a nuke your way and cover it up with politics (which is why being a politican helps a ton so you understand how these things work).
Hisoka is the most successful example of a bad guy who managed to make a name for himself on just brute force alone. But even he operates under the laws of HXH. He become a Hunter to avoid dealing with the Hunter's Association in the first place. He is aware that Hunter's Association forbids 'heinous crimes', whatever this actually means, and we can see that for the most part Hisoka avoids taking action in massive murdering sprees without reason. In fact his biggest murdering spree by the numbers was done during the Hunter Exam in a portion of the course where your average guy isn't expected to make it out alive anyway. He took almost no part in the Spider's rampage in Yorknew against the Mafia. The only time he violated this principle was in the Election Arc, but even there he made sure there are no witnesses (no party seems to be even aware that Hisoka was involved), and of course in that arc he was fighting to protect his own life since he's under the impression that he'd be dead Killua had to repay his wishes.
Although Hisoka's murderous actions are well-known, he's never done anything that'd cause him to get outlawed from the Hunter's Association. In some sense he is no more powerful than an organized crime lord, which can coexist with Hunters. As long as you don't do whatever Hunters deem as 'heinous crime' they'll leave you alone. Hisoka covers his track well and has even plausible denialability in most of his killing sprees.
Of course the Spiders are also successful too, but they've massive numbers (13), while the largest organized group of pro Hunters we've seen so far is 4 (in GI) (not counting the group of useless guys that were killed in the Election Arc). Although more than 4 Hunters were mobilized to deal with the Ants, it was more like 3 teams of 2 + Netero. Hunters do not operate in large groups by nature so the relatively large numbers the Spiders enjoy contributes to their longevity in a world dominated by good. Even then, the smart Spiders keep a low profile. In group missions, Kuroro always has at least 2 other members by him, and he disappears after a mission is done. When Machi contacted Hisoka it's clear she avoided any exposure longer than necessary. Even in Yorknew when the Spiders are relatively bold, we see that Kuroro sent 2 more guys to follow Machi and Nobunaga in secret.
So going back to the original topic, asking whether there are strong people in the Hunter's Association is pointless. The Hunter's Association is supposed to be overwhelmingly powerful to the point that no overt form of evil can possibly exist in the world of HXH. Every character of dubious character either operates from the loopholes or has some kind of political protection, because if you're stupid enough to openly defy the rules set aside by the Hunter's Association, it is a given you'll be ruthlessly crushed. By the way, being an Assassin is clearly considered a normal job in the view of the Hunter's Association. After all Hunters can work as hitman for Mafia bosses so why not Assassins? As long as you're not killing indiscriminately enough to violate the code the Hunters don't really care about other people dying.
To stay on topic discussing power in HXH is difficult since Nen is sort of difficult to encompass in a normal battle system-type thing. Especially since we have things like Alluka that can (at least at this point in time), take away the repercussions for using up your nen to go Hulk on your enemies.