Since God's command to not murder is also quoted in the New Testament (Mark 10:19 is one example), it's not necessarily inconsistent with their position for them to quote that command.
However, if they believe the New Testament, you could ask them if they believe what Paul wrote in 2 Timothy 3:16-17: "All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work."
Paul wrote "all Scripture," so anyone who claims to believe the New Testament should honor the Old Testament too.
Christians shouldn't discount the Old Testament, since it's God's word.
(Hope you don't mind the comment.)
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