My sympathies.
In order to learn from this, you have to analyze why the fish perished and do not repeat the same error.
I don't see any obvious or bad damage to the fins you are describing. I don't think it was the direct cause of death. The stress of being harassed by a tank mate could have been it. If it was 3 days of continuous or regular harassment, that plus the stress of transport and acclimation to a new tank could have pushed your arowana over the edge. Stress is one of the top killers, if not the top killer of fish in our hobby.
If it was a blow to the head, which often happens when arowana try to jump to escape a stressful situation, you'd see obvious bang mark and bruised mouth and hear area. If you don't see it, it was most likely just stress.