Since Skin Cells Have Human DNA, Would Killing Them Be Murder?

Questions about whether killing skin cells (which have human DNA) is murder, how to know when to stop engaging someone who is antagonistic, how to graciously decline to engage in a discussion with someone who just wants to argue, and what to do when you hear Christians misinterpret and misquote Scripture.

• If a human being begins at fertilization, and killing an embryo would be murder, then when I scratch my skin and kill human skin cells (which have human DNA and might develop into full human beings through cloning), is that also murder?
• How do you know when to stop engaging someone who is antagonistic?
• How do you graciously decline to engage in a discussion with someone who just wants to argue?
• What do you do when you hear Christians misinterpret and misquote Scripture?