If you take away the regular good citizen's right to arm themselves for the very fact that a freak who's going to break the law to use one... Would be an idiotic and purely stupid way of going about things...
The lawbreakers would always be able to get ahold of armed goods anyhow since that's their goal in the first place is to break the law.. And they'd go to any length necessary to do so...
Leaving even more casualties in the wake for a regular person to have a lack of defenses at their disposal... You'd basically be creating open season on every civilian out there... Since the only possible deterrent for a criminally active mind is the fear of the unknown that their victims might be armed.. That they too might die..
The only difference in this case being that the assaulter was suicidal so had no deterrent of that in their psyche...
Your point is invalid and a stupid one at that...