something like a vigilante, eh? Well, let's see.... there really is no one answer, because the possibilities are so vast unless you specify further. This Pokemon will either be taking out humans on its own, which requires skills, or it could be leading its own army of other Pokemon, which requires leadership (much like Mewtwo was attempting with his army of clones), or it could do both. Whatever the case, the Pokemon should be considerably intelligent.... although, it's kinda hard to distinguish the intelligence of certain species. We know Alakazam's smarter than a super computer, but then again, Pokedex entries shouldn't be taken to heart too much, since they talk about how Dragonite and the Latis understand human speech. That's an inconsistency, considering we all know every Pokemon in existence has at least enough intelligence to understand commands. Although, it might look a bit awkward to see dopey Pokemon like Numel or Slowpoke doing anything more than following orders.
Should it be taking out humans by itself, it's gonna have to be strong. If humans are wiping out Pokemon, they WILL use technology to accomplish it. Be it realistic destruction vehicles, or the numerous gadgets that Team Rocket has, they are bound to use it. The Pokemon's, of course, gonna have to be strong enough to take down such vehicles, or at least be strong enough to break windows or doors and take out the driver, maybe even just lift off the panel to a control circuit (if not, tear through it). The Pokemon should be very mobile as well, capable of reaching high places (like the afforementioned driver's seat to destruction equipment, or control towers), so either flight/levitation, climbing, or clearing tall jumps is essential. But more often than not, at least one of those (tall jumps) will be covered by another requirement, and that's agility. Humans use guns, the Pokemon would have to dodge bullets somehow. Though, alternatively, the Pokemon could just have a high defense and take those bullets. Golem, Steelix, Aggron, etc. don't seem like they'd have much trouble thrashing destruction equipment, neither. They're capable of climbing, too, but it's not like it'd be too necessary, considering they're probably strong enough to take out such things from the base. However, when it came to sea-based raids, like sneaking onto a supply ship or getting to an oil rig, those Pokemon would have some problems.
If the Pokemon's leading an army, though, pretty much anything is possible, because even if the Pokemon is considerably slow and weak, like a Caterpie, yet manages to provide intelligence strategies to its army, that shouldn't even matter. They'll likely have some personal bodyguards that do fall into the above categories, and both transport and protect their leader. Water Pokemon that are limited on land could also have psychic Pokemon that carry them around.