"the jurisdictions with the strictest gun control also have, in general, the most violent crimes committed with guns per capita."
That simple correlation does not support the argument that "When criminals think their victims are likely to be armed, and are fully capable of defending their own, they think twice about committing crimes there". An alternative explanation might be that in more dangerous places with most violent crimes committed using guns, the people there voluntarily choose to impose stricter gun controls. That would not mean that gun control did not work either. It might be that the effects of imposing gun control takes slower to realise after having a long period of loose gun control laws, or it might be that the benefits are limited because people can get their guns easily in neighbouring states since there is no blanket ban. In this case, imposing strict gun control over the whole of America would be successful in reducing violent crimes committed with guns.
What about the case of Australia?
How many mass shooters have been stopped by victims or by-standers carrying guns? Florida, where the latest mass shooting happened, allows for people to carry guns.
The case of Australia is irrelevant because it's not the United States. What about Japan? What about Russia? I mean we cannot look to any other country when determining effectiveness of a US policy. We can only look at the US and see what happens when policies are tried in the US.
Gun control doesn't work in the US. Possibly because there are too many guns, along with a gun culture. Australia had less guns, did not have a gun culture, so that could explain why their policy had different effects. It's also true the war on drugs is effective in China, in Japan, in other countries. In Asia in some countries you get the death penalty if you're involved in the drug trade so I guess this means the death penalty works because it worked in Asia? No, Asians have a different culture and what works in Asia might not work in the United States.
Either show the data in which case it is a fact or this is your own opinion not based on data or evidence. Gun control already exists and it's not having an impact in the data. Gun control would not have prevented Cruz from getting his gun legally and if you think it would have how about explaining how you think it could have stopped him?