Sometimes cats just disappear for a few days at a time. It is possible that dogs could have killed it, but it also could have just decided to go exploring, or gotten stuck in someone's garage over the weekend while they were out of town, or anything along those lines. Unfortunately, it's also possible that it got sick, or was hit by a car, or was injured and unable to make it home.
Bottom line is, you don't know what may have happened, and hopefully she will turn up soon, but if she doesn't, you can't really go kill animals that might have had nothing to do with it. In addition to possibly wrongfully killing an animal, you could end up in all sorts of legal trouble if the dogs actually belong to someone, because that would be destruction of personal property. And depending on where you live, there may be ordinances against firing a weapon withing certain distances of residences, and/or killing an animal for a purpose other than state sanctioned hunting or self defense. You should make signs with pictures of your missing cat and call the local shelters in case she was picked up; and call animal control about the loose dogs.