In a peaceful, balanced, anarcho-capitalist society, is it a violation of the non-aggression principal or permissible to kill a neighbor in self-defense because they are financially supporting a warlord? The warlord is moderately distant and doesn't pose an immediate threat, but commits atrocities and violations of human rights, civil rights, and the non-aggression principal. The warlord gains followers and power, either through charisma, promises of spoils from plunder, a claim of spiritual knowledge, etc. If the warlord's power grows, they may attack your security, your city, or you. It will cost you more money to pay for heightened security and defense at the very least. What are the options for such a situation and what is permissible?
A basic principle of self-defense is that you should answer with similar force. So if one person is trying to kill you, you can kill that one person. If a group is, you can try to kill that group. But you generally can't kill a group because one member is trying to kill you (exceptions may apply).
In this case, someone is doing you financial harm. So respond financially by suing. If your neighbors are causing you financial damage, then you should pass your expense to them. That ensures that they are bearing all the costs of their decision as well as the benefits. Note that the lawsuit may be difficult, as you would have to prove that your neighbor's financial contribution to the warlord is causing you financial harm. Your question implies that that is possible, but a court might disagree.
If you look at the Wikipedia article for the non-aggression principle, you will see that they discuss a similar problem with pollution. Your neighbor causes pollution without your consent, damaging you. You have the right to sue for compensation for those damages.
Local law may matter here, but the principle is rather basic. Local law is unlikely to allow you to kill someone for a financial action. The lone similar example that comes to mind was the weregild system, and that's pretty much the reverse situation. A murderer could make financial restitution rather than be physically harmed in turn. And of course some legal systems allow imminent theft to be met with deadly force. But this is no direct theft and isn't really imminent either.
You need to know why that neighbor is paying the warlord. Most people aren't main players in a case like this.
Example: Armed conflict between A and B. Both get help from local populations because the latter know there would be retaliation were they to object.
Both A and B punish the local populations when word gets out they're helping "the other camp". From their perspective, they're "betraying" them. The local populations didn't choose to be involved, they're between a rock and a hard place.
Which brings us to your neighbor; one needs to figure out a couple of things: is he really financially supporting the warlord and how do you know? If you had access to that information, who else had? (i.e: who else knows that neighbor's doing that?). Is it a fact or a rumor? Is it hus-hush or explicit? What's the neighbor's reaction to his "reputation"? Embarrassement, arrogance and bullying? Is it his money he's giving or is he a relay? If he's a relay, has he ever bragged about taking money from the warlord? Does his lifestyle indicate he has more money than other people in his position?
If one wants to take that financial aid out, they could make it possible for evidence to reach the warlord to prompt some house-cleaning.
Why he is financially supporting the warlord. This will hopefully give someone an angle from which to maneuver and an idea on which incentive to present the neighbor with to manage the situation.
Is he doing so out of fear? Is he related to the warlord? Blood or marriage?
What would happen if he stops paying? Has he ever "defaulted" and what happened? (what's the warlord reaction).
Were he to stop paying, which kind of visit will he receive? (at that point, it will probably escalate and they'll try to know why he stopped paying and will most likely know which external actor weighed in. The external actor will most likely be attacked by them and in the case he was interested in "self-defense", he has it now. Is the external actor ready? How doable is the whole warlord's group extermination? Do they meet in the open? Do they make any effort to hide or do they love to have a name and be seen?).
How significant is his contribution as a percentage? (if he's responsible of 90%, then changing his mind will be a really hard blow for the warlord).
Is there a time in year where the financial contribution matters most to the warlord? (i.e: what's the relative value of each dollar throughout the year?). One would gain disrupting the money flow at a time where it's most needed (low reserves, warlord owing money to someone, henchmen haven't been paid in a long time and extending that time by cutting the money flow, but that's the kind of thinking that made the Middle East what it is today. The warlord isn't anymore, but now it's chaos).
Anyway, this is just some things to consider. The original question hasn't been answered because one don't have to think about it that way. It's been asked that way as a means to an end in the first place so it's not about the neighbor, therefore other options can be addressed. And having established this, maybe other means can be found.