Basically the scripture about the killing of non/un/incorrect-believers refers to the context of idolatry in the Bible. There is a thin line that God draws between those who have belief in him, and those who don't and preach the wrong diety. There are a lot of verses in this regard about preaching to the "wrong entity". It is heavily looked down upon and not accepted by God.
Revelations in the NT emphasizes on the judgement of incorrect followers, greatly, by referring to someone who appears to be an anti-Christ that emerges in the new age (that we are currently living in). The NT conveys that people will speak before they think, and there will be those who follow the false figure/prophet. On what appears to be Judgement day, each and every of these followers of the false prophet will be cast into the lake of fire and endure their punishments for a long time.
Different sects of religion interpret the meaning in their own ways. You may notice that some people/churches have the jewelery, crosses, and paintings. Some do not have paintings, but have the holy cross or wear jewelery. Some choose not use any religious symbol at all.
Because, these are all created in an artist's best impression, to which might not symbolize the real thing depending on how the sect views it.
Just a little insight. I'm not advocating one or the other for anyone, just clearing the message so that it is understood.