Actually Tinny, you are describing an excuse
for the contradiction between the old and new testaments, not a valid reason
. You're citing a theological cop-out.
The actual REASON that the bible contradicts itself would seem to be that the teachings of Jesus- a proto-moralist philosopher- have no place alongside the blood and gore of theistic judaistic dogmatism, whether found in the old testament or the new.
The fact is that the old testament is broadly speaking a selection of borderline insane ramblings about a perpetually disgruntled and extremely vindictive judaistic weather-god... and the bulk of the new testament concerns descriptions of the life and times of a peace-loving dissident who may or may not have actually existed. (But wasn't a divinity.)
The two do not belong together in philosophical terms. So frankly, save your intra-christian theological excuses for your debates with the fundamentalists!
It is sufficient for we atheists to note that there are some christians like yourself who believe that the new testament teachings of Jesus override the extremely objectionable mosaic law... but there are plenty more christians who swear by the old testament and all its fire and brimstone! Especially in the US. All those evangelical cultists.... ugh.
You might claim that your point of view represents "true" christianity, and I might agree with you in some philosophical sense... but that doesn't change the fact that a great many people who call themselves christians still consider old-testament guff to be valid. And this belief contributes to the character of the organised religion we know of as "christianity". End of story.