Killing innocent being (irrespective of muslim or non muslim) is one of the biggest sin in Islam. killing one innocent is just like you killed whole of humanity.
Quran says in Surah 5 verse 32
"On that account: We ordained for the Children of Israel that if any one slew a person - unless it be for murder or for spreadingmischief in the land - it would be as if he slew the whole people: and ifany one saved a life, it would be as if he saved the life of the wholepeople. Then although there came to them Our messengers with clear signs,yet, even after that, many of them continued to commit excesses in theland.
Hopefully , these verses of the Quran clarifies you to whom we should fight and whom we cannot. We should pay attention to those people who just quotes thing out of context and some time wrong text and try to say that Islam allow to kill innocent non muslim. Islam does not allow to kill innocent peoples whether muslims or non-muslim , and teaches us to treat them well.
