I understand Christianity as a NEW Testament or Contract if you like.

A New contract makes the the Old contract null and void.

Christianity should be under the teachings and words of Christ IMO.

Violence? What does Christ say about it?

"Turn the other cheek"
"Love your enemies"

And how can I explain the killing and violence in the Old testament?

I can NOT. I do NOT understand these things. I also do NOT understand
why the dinosaurs were made to be extinct.
There are many mysteries I do NOT understand.
Nor do I need to.
It is enough just to attempt to follow the WAY of Love.