The only thing the landlord can do is EVICT the criminals (if they are residents) and there's not a single thing the landlord can do about other criminals that come on the property.

thanks.Did he press Criminal charges?

I don't mind oil companies getting a few breaks for hiring people, but what they get away with is almost criminal, certainly not in the national interests but thier own.

My major is psychology and my minors are forensic psychology and criminal justice.

And 20k, no offence is not that much (even though it may sound like a lot to you, in 5-10 years from now you will regret filing over 20k) and if you were asking most investors many are 20k in debt and then some when starting out.

What is the current law on refusing to rent to prospective tenants who have criminal records?  

I'm sure he knows a bit about criminal law. 

Not to mention, there are potential criminal penalties.

There are a lot of programs and softwares you can use that will screen your tenants at no cost to you - the tenant will pay the rental application fee and you as the landlord will get a criminal background check and credit report etc.!

Running criminal background is an absolute must.A $12 criminal background check might have kept this killer out.I am going to point out, in the landlord's defense that it is possible that he didn't rent to this tenant.