Definitely agree with the dangers of over-leveraging.

I then immediately changed the locks.This person was not dangerous, per se.  

You also stand a chance competing in on-market deals where a seller would prefer not to deliver vacant (remove existing tenants, e.g. for a developer to start work to condoize immediately), if you're willing to take a risk on taking them on.Also, keep an eye on Malden and Chelsea - lots of two-family turnover in the past 2-3 years in Malden and it's been hot, fewer three-deckers there, but still room to work as a house hack on the orange line and commuter rail, and Chelsea JUST got silver line access this year which opens up some three-deckers that were previously less accessible (and accordingly, less rentable).Happy to chat more if ever it's helpful!

And even fewer have a personal background in investing.We have close to 5,000 licensed agents in the Milwaukee metro area, which is absurd, because the vast majority does 0-5 deals a year. 

I do not want to have a thief living in my house, especially if they are a danger to my other tenants.

Property prices and rents are both driven by supply and demand.If there are more sellers than buyers, prices will fall.If there are more buyers than sellers, prices will rise.If prices rise, fewer people can afford to buy and will be forced to rent.

These types of investments are considered dangerous by traditional mortgage lenders, banks, and credit unions, who shun them.

This is dangerous ground to tread on at this point. 

Nuisance would include creating danger to property and other residents.

What can make it dangerous is when something occurs to cause it to grow outside of it's normal levels.There are swabs for surface mold for testing and air tests for airborne.