It’s not so much that our values are inverted as that we are desensitized to human killing because we watch it so regularly. Pet killing is rare enough to trouble people enough that it isn’t often shown—which makes it continue to be rare...
Everyone likes murder movies or war movies or cowboys and indians. No problem seeing people die, it’s an everyday occurence in real life. Who would want to watch a movie where pets get killed? What would be the plot?