Ryan, just to be clear, I am in "violent agreement" with you.No one's paying you or me to post on these forums.

We discvered that violent gang members had inhabited the house for well over a year.

Do they only check for violent/sexual crime and threats to other tenant, or is a marijuana felony usually enough to keep a tenant from renting a place to live?

No drug or violent crimes.

Police can't chase down every suspicious lead from people untrained to categorize a person with mental health issues as violent or not.

If an applicant or tenant moving in expresses concern about wanting to lock in longer, I assure them that the only reason we serve notice is if they aren't keeping up their end, and we do not frequently or violently raise rents.  

I mean are they a violent criminal that you fear will vandalize the place or accost the tenants?

The reason for an address which isn't knockable is due to the violent nature of some ignorent people.

A personal favorite of mine, Albert Einstein's well-known quote:Great spirits have always encountered violent opposition from mediocre minds.

http://www.redsticknow.com/violent-crime-homicide-baton-rouge/