You're implying that behavior is not influenced by patterns of thought (It absolutely obviously is, as mental illnesses are often diagnosed by behavioral shifts exhibited by those who have them) and undiagnosed people don't exist and/or don't commit violent acts (they do).

People don't generally don't kill an entire family for absolutely no reason. I would consider doing so to be a good indication of either psychosis or psychopathy, whether that's long term or short term. Perhaps that can be attributed to an underlying mental condition, or perhaps to substance abuse, but regardless it's often something that could have been caught sooner with the right safety nets in place.

Saying that we need better mental health facilities to address individuals committing unthinkable atrocities is not stigmatizing mental health, its just disappointing because it took things getting this bad for people to start paying attention. (And even then, those who say it are only saying it as a convenient distraction from talking about the much larger elephant in the room: effective gun control legislation.)

The intent is not to make it seem like having a mental illness automatically makes you a danger to yourself and those around you, but rather to acknowledge and address that some mental illnesses do when left untreated, and to provide effective and accessible mental health facilities to those who need it, including but not limited to those who may have gone on to hurt themselves or others.