It increases the odds of a shootout. It increases the odds you get shot. (Maybe he's a better shot than you. Maybe he approaches you with it drawn while yours is still holstered. Not much of an edge for you.
It increases the odds an innocent bystander gets short. It increases the odds of an accidental discharge...
I'm Not saying I wouldn't want to have a gun in my hands if I were attacked, but its wrong to oversimplify it so that is the only scenario we look at.
It is evident you don't know the psycology of a rapist, or indeed most criminals.
Most rapists, in particular, seek to exert power and control over their victim. The best way to assure success is to target the most vulnerable, because rapists are cowardly and are afraid of failing to obtain that control and power (often they lack power and control in other aspects of their life and are loathe to face that failure when they don't have to).
Rapes are virtually unheard of in public places--they happen at night or in secluded places--the chance of an innocent bystander being caught in crossfire is basically nil in those cases.
Rapists don't tend to be gun enthusiasts--they may brandish a gun or a weapon when they commit the offense but the purpose is to intimidate and control; they generally lack the courage to discharge the weapon. A dead victim can no longer be threatened or intimidated, and if they've gone and shot their victim they've lost control of the situation. If a potential victim were to pull a gun in the vast majority of cases they would flee, not fire back. If the odds are against the attacker they will flee. Rapists who are armed almost NEVER approach with a gun drawn--they will only brandish the weapon when they are very close.
The studies cited about an increase in violent rapes and assults in Australia is not really surprising at all knowing this--it isn't specific to Australia's society--though different societies would respond differently to changes in criminal laws all rapists share some common traits, and the more confident a rapist can be that a victim is defenceless the more likely they will attempt the crime.
The kind of situations you describe, where victims have their weapons used against them or are bysanders caught in crossfire, mostly happen as a result of organised crime or street gang activity. The target and the perpetrator are both criminals, and both are probably armed with illegally obtained weapons, and the motivation is not control or personal gain (like robbery--motive is to obtain something of value not to kill). Gang members kill each other out of revenge--to settle a score. Such people wouldn't go through the bother of using a 3d printer--they have their sources of illicit weapons already.
The most visible, but most rare as well, victims of gun violence are those of the emotionally disturbed, generally suicidal deviants. They are very rare cases actually, and if a bystander was to fire back the situation is proabably already exceeedingly dangerous already.
Gun control is treating a symtom generally--it is not very effective. Cun-making-control even less so. Treating the causes would work better but is more difficult and less politically expedient. Those causes are many and range from urban blight/decay to public school systems/modern "self-esteem based" teaching philosophies that foster sociopathic behaviour in children to family breakdown to lack of comunity resources to help raise children (and as a result are lured by gangs). Gun violence is a complex problem with no single easy answer. Unfortunately the media advocates quick, easy answers and people demand them, and ultimately laws are crafted on that basis.