There are not an insignificant number of cases where a normally responsible person becomes an irresponsible person,
When we're talking about irresponsible enough to commit homicide, yes, that is an insignificant number. (In terms of frequency; of course in personal terms any murder is highly significant to those, to friends and family of both the victim and, in a different sense, the murderer.)
Murder is something people work their way up to. 90% of murder suspects in Milwaukee in 2001 had a criminal record; the same proportion was found in NYC in 2003 through 2005. Keep in mind this is just guys (mostly, some women too) who got caught at previous crimes, more would have committed crimes and not been caught, and more would have displayed irresponsible but non-criminal behavior (the sort of stuff a good mental health system would catch).
The good citizen who suddenly snaps and kills is a favorite fictional trope, but bears little relationship to criminological reality.