If the landlord received the funds from the check with he cashed (that was made out to your roommate), then your bank is on the hook as it was a forged endorsement. Immediately contact your bank. They will probably have you sign an affidavit, and they might also require you to report it to the police.
The bank should reverse the transaction and make you whole, so there won't be a loss to pursue against the landlord.
Even if the bank does not require you to report this to the police, I suggest that you do UNLESS you know that his submitting this to the bank was just an accident. For instance, if the landlord manages multiple units and only heard from you after endorsing and submitting this check with a bunch of others, it's possible that he didn't even notice it. It's also possible that the person who opens the mail to process the checks isn't the same as the person who endorses them.
In any event, I can't imagine how this would give you a legal reason to get out of your lease. If you are legitimately concerned about your safety or security, then it's worth looking at the lease and maybe trying to find a way out, but that seems unlikely to me. Without knowing more, it's hard to say. For instance, is the "landlord" the owner of the property? A manager for an owner? Are you in a large development? etc. It might be that there's someone for you to complain to about your security concerns, and that would get this guy out of your life.
If not, take a look at the lease based on the facts at your disposal. It's worth a shot if your safety is in question. Check out your "quiet enjoyment" and other similar sections. In any event, this likely will require written notice. Read carefully and follow the contract to the "t."
-- Loretta Salzano