It’s great that public opinion seems to support universal background checks for gun ownership. It’s unfortunate that background checks are futile in controlling gun violence.
The flawed assumption is that background checks can determine who will be a "good guy" in the future. Clearly they cannot.
About 80 percent of weapons used in mass shootings were purchased legally, according to many different sources. In other words, we are not good enough at judging whether current law-abiders will remain so.
We all know cases in which perfectly nice folks went over edge due to nasty break-ups, job loss, drug addiction, isolation, radicalization, personality flaws, whatever.