that's normal behavior, can't change them . Big tanks , big tank mates, decor to break line of sight etc are all great suggestions for decreasing/slowing down the kill rate but in the end they are a large and powerful species that is very territorial/aggressive it'll likely kill any tank mate eventually.
my silver arowana was surplrisingly mean to my baby cats when I gotem, then I add two (larger) Oscars and they put him in line. havnt had any problems since.