Can genocide in the bible be justified today?
I don't think so just like genocide in current newsprint can't be justified today.
There is actually a wonderful YouTube video by DarkMatter2525 about this very question. It is an animation about a pastor in a church and a man who appears claiming to be a time traveler. I don't want to spoil it for you so I am providing the link. It is a great story narrative with several twists that helps answer the question in an entertaining way.