1/06/17 update regarding comments and other answers:
You can report criminal intent while reasonably remaining anonymous by using various methods, and this will let you do want to do: take an ethical step and make an effort to report the potential crime. Read below.
First of all, using Tor is not illegal. Yes, there is illegal activity on Tor and the dark web it can access, and there is activity that promotes illegal things on Tor and IRL, but that doesn't make you a criminal by default by using a system that can be used by criminals. See Is it legal to use the Tor Network and Tor Software in the United States?
The FBI and other law enforcement and intelligence agencies are well aware of Tor and the dark web; see https://en.wikipedia.org/wiki/Silk_Road_(marketplace) . (That may give you pause because some FBI agents were implicated in crimes when the agency investigated the broader crime.)
Or should I assume that the proper authorities are already monitoring
I wouldn't assume they are already monitoring that particular site, but it's possible; the FBI at least is aware of its existence. It's a good idea to assume they run their own crawlers to index the site, and have agents who are members. Think of all the news stories your read about law enforcement posing as perps online to ensnare people; and then think about how much there is we don't know about these types of activities and their prevalence.
If you really want to be paranoid and know that Tor is not perfect security, and that government agencies have serious power, know that the NSA is well-known to run many Tor exit nodes to packet sniff and store metadata and the full text of traffic (as well as clear, non-dark web traffic, too). And, it's an accepted fact that SSL has been cracked (SSL may or may not be in use in your case or technically relevant right now) by them, too, all re: Snowden.
Deleting Tor from your PC/Mac means nothing; any 5th grader can do forensic analysis and discover the remnants of the Tor browser. Your local ISP may log Tor traffic, as such traffic has a signature. But again, simply using Tor doesn't not make you a criminal, but it could draw attention to you. And warrants get be gotten for ISPs and your PC.
You remaining anonymous overall also assumes that you have not given out any information on that SE clone site that could identify you IRL. If you have, then you need to assess the risks to yourself; that could be your most serious privacy breach to other users beyond any technology (as well as to law enforcement if they for other reasons suspected you of illegal activity yourself.)
That said, if you want to report the possible crime - for either legal or ethical reasons - you can remain mostly anonymous via email.
Use the Tor browser from public wi-fi (try on a USB stick on a public PC and not from your laptop) and sign up for anonymous email with a fake name (breaking their TOS) at Yandex, which does not require phone verification. (Google, Yahoo, etc., require a phone number; this could be a burner or IP phone, but that's up to you). https://www.eff.org/deeplinks/2012/11/tutorial-how-create-anonymous-email-accounts
You could also use one of the various disposable email sites, as pointed out by another user, but some such email domains and services are often automatically flagged as spam or blocked by agencies due to their nature. Try Google for "disposable email".
Ditto the anonymous phone call services or email-to-fax services. There are different methods with different anonymity pros/cons.
And, you could use snail mail, as was pointed out by another user. Mail a printout in a plain envelope (with no fingerprints) in a public mailbox some distance from where you live and usually use the PO; the USPS records metadata on all mail, but this can be reasonably anonymous, with care.
In any case, since you read that the person has already "cased the house" and have a general geographic location, the FBI may take the information (and your tip) seriously and look into the possibility of a crime. If the FBI looks, they may not deem it serious enough - or see enough evidence - to do the work required to find the true IP or other digital forensics to locate the person.
I don't know if it would make any difference (because of Tor's
But we don't know the other digital breadcrumbs the alleged perp has left; it's possible they could be tracked more extensively online - and then possibly located IRL - by the FBI.
And, if you do send an anonymous tip by any method, you've done what is appears you want to do: take the ethical step and make an effort to report the potential criminal situation.
1/05/17 original answer:
Take a look at FBI Forms — Tips - fbi.gov to submit an email tip:
Please use this website to report suspected terrorism or criminal
activity. Your information will be reviewed promptly by an FBI special
agent or a professional staff member. Due to the high volume of
information that we receive, we are unable to reply to every
submission; however, we appreciate the information that you have
And/or there are phone numbers here: Submit a Tip — FBI