Why can't you? The bible is nothing more than a bunch of short stories in a compendium. Some of them are factual, some of them are fictional, some of them are op-ed, some of them are straight historical documents.

Cause it would defeat the purpose of believing in the Bible to begin with. "Hey, I don't like that part in there where it says I can't kill, so I'm just going to ignore it." According to your logic, he would still be a Christian and he could go around killing people. It even states so in the Bible "Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the Lord your God which I command you. " Deut. 4:2