Well, "Shalt" is an out-dated version of "Shall", which was used for second-person singular 'Thou' (Thou means You). And "Thou Shalt" means "You Will/ Shall". So, your sentence, 'Thou shalt kill', means 'You will kill'. But as mentioned that it is from Bible, I'll say that you've read a misprinted Bible. The actual sentence is "Thou shalt not murder." meaning "You shall not murder."