I have seen swat teams raid neighboring units managed by slum lords who didn't screen tenants, I have dealt with abusive boyfriends putting my tenant in the hospital, broken down cars (which leads to them missing work because they can't afford to fix the car, which then leads to them loosing their job and then getting behind on rent), being threatened with a gun (guy lifted up his shirt showing me the gun in his belt), to countless sob stories. 

Now after couple months he send me an email and demand about 3800 worth of plumbing repairs and threatens me to sue.

Rent is always paid in full but the agency, if the tenants screw up, I can threaten them with calling the agency. 

The tenant is now threatening you with the system it sounds like he may have some knowledge of.

I currently hold their security deposit, only $500 and technically I have threatened to report their business to code enforcement before (I held off) but I could use that as a negotiating tactic for leaving on time, since the fines are up to $5000 in Miami and it’s possible this tenant Does not have full citizenship status. 

I certainly don't wish to threaten them even though they don't seem hesitant to do that to us.If they don't move; can I sue them for damages as a result of losing or inconveniencing the other tenants?

unfortunately it's a case of sellers remorse, where the seller doesn't really sell for the amount someone had put in their head that their house was worth, I think it's taking it pretty far to threaten to take legal action, what do you guys think we should do?

I finally got a copy of the lease after threatening to not close this week.

So what was originally projected to be a solid return is threatening to be a huge loss if the market tells us we are overpriced.  

She also threatened the manager with bodily harm, and had some of her "thug" friends show up at the property to intimidate.