Ummm, so please provide me a Web link or online resource that instantly shows his criminal background?
You say that it "only took people minutes after learning his name to know he had a violent criminal history", but that is because it was publicized by the media and law enforcement AFTER his name was publicized and AFTER the crime was committed. If you do not work in law enforcement, such information can be obtained by paying a fee to various online sites that provide background checks (e.g. intelius.com , beenverified.com , instantcheckmate.com). But she may not have been aware of those resources, and most people probably either are not aware of those resources, do not care to pay the fees involved, or do not remember to do it. Ten years ago, marriage and divorce records were freely searchable on the Web sites of some state governments, but even those are now no longer openly available to the public. Criminal records can also be expunged or sealed, and visible only to LE. More people remember to ask their partner for a blood test as proof that they are free of disease prior to sex than to perform a background check on a new partner, but even that is often not done and people just ask for a simple yes/no verbal confirmation that their partner is disease-free without asking for proof. I think that I am very good at cyber-snooping people, but excluding the publicity about the murder, I only found JC's Facebook page and later found his Plenty-Of-Fish and Mingle dating profiles. Just from his Facebook page, he seems to be very close with his sister and not that abnormal.
Ideally, every person who dates someone, regardless of whether they are male/female and whether they met online or in person, should perform both a background check and request official proof of being disease-free from a fairly recent blood test with each new partner, instead of just asking if they have ever been arrested and just asking if they are disease-free, but the vast majority of people do not have verified proof about their partner's arrest and disease backgrounds. Have you asked every previous partner for verifiable proof of their previous arrests and a copy of a recent blood test to check if they are disease-free? Have you even at least simply asked every previous partner if they have ever been arrested and if they are totally disease-free (even though they can lie about that)?
Being too naive is never a good idea, not just "in the world in which we live today", but also more than a century ago when Jack the Ripper roamed the streets of London.