Actually, it says "Thou shalt not murder."
לֹא תִּרְצָֽח׃ ס
(ratsach)
רָצַח râtsach, raw-tsakh'; a primitive root; properly, to dash in pieces, i.e. kill (a human being), especially to murder:—put to death, kill, (man-) slay(-er), murder(-er).
Murder vs. manslaughter is very clearly defined by Moses (Num 35) in the same law which recorded "Thou shall not murder."
So, obviously, it was defined at that time.
Even by your own definition it's 'kill'. Saying 'especially to murder' doesn't affect or change or negate the first part which says, 'kill (a human being). Basic English comprehension. ie. I love all animals, especially cats, doesn't change the fact I love all animals.