But perhaps the idea is that of deterrence, i.e. that you don't actually plan to use the weapons unless someone forces you to use them.

See Mutual Assured Destruction.

Or you can use them defensively. Killing is evil, but if someone points a gun at you, would you still feel evil if you killed them instead?

Weapons can also neutralize weapons (instead of people), e.g. missile interception.

Further, it might be possible to develop effective non-lethal weapons (military equivalent of taser).

Finally, developing weapons can also mean that weapons become safer. For instance, less collateral damage. Or weapons that can be controlled better, so they can't be used by people who shouldn't use them.

Do you believe that having a military is evil? Serving in the military is evil?

That's an intellectually dishonest question and you know it, you ought to be ashamed of yourself.

How? It's a legitimate extension of the previous thought. If it is viewed as evil to develop instruments that would kill people, is it also evil to use those instruments to kill people? Likewise, if no one developed instruments to kill people, the military would be an ineffective organization at the task that it's responsible for, and thus would the organization also be considered evil?