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.
