Trouble is, with the threat of litigation being so great, you need to be damn sure that 'taking the shot' to curtail an attack is completely justified.
The risks of then being shot or shooting a bystander notwithstanding.
It's a poor analogy, but at work, we have a permanent team of highly trained fire fighters with the best equipment who can get anywhere on site within 2-4 minutes. In spite of this, everybody on site has to have compulsory annual fire safety training which includes selecting and using a fire extinguisher... With the caveat that we shouldn't tackle anything beyond an office waste bin. Ergo, in spite of training, walking away and raising the alarm is always the preferred option...
Removing the Second Amendment will, and should never happen. However licensing of gun ownership could be done in a similar way to the issuing of a driving licence, which in the States is generally a staged process. This way, you could only graduate to more powerful/dangerous weapons with formal, regulated training and professional supervision.
Won't stop these types of incidents from occurring, but it might help educate the broader population...