Common misconception, thanks to the all-too pervasive influence of the horribly inaccurate King James translation: the word used in Hebrew does not actually mean "kill". It's more akin to "thou shalt not commit murder" or "thou shalt not kill without justification". Of course, there's a whole new can of worms opened up there to debate whether any killing is justified or not. The Bible itself gives a number examples of when killing could be lawful - capital punishment, war etc - and you'll find much more to read in both Jewish and Christian philosophical traditions. A superficial dismissal based on perceived hypocrisy, however, doesn't stand up to even the most cursory examination.