Will this incident encourage more criminals to carry guns? Probably because they will feel the need to protect themselves against the possibility of encountering armed victims. So this so called good Samaritan may be creating more problems than he solves in the long run.

One reason that the UK has a low homicide rate is that criminals know that their potential victims are very unlikely to be armed so they don't feel much of a need to carry a gun. So less people end up getting shot in confrontations with criminals.

This is one reason that arming law abiding citizens will never deter crime in the long run. It starts an arms race because criminals feel more of a need to arm themselves if they know they are more likely to encounter people who are armed. And criminals have the advantage of surprise so a gun benefits a criminal more than a law abiding citizen in a violent situation.
Progress is being able to admit that you are wrong.