Re: Lump on side of trout
Let's presume that this fish was one of many who were recently introduced and was reared in a hatchery. Fish, like people, can and do develop various diseases and those fish who are grown in the crowded and often dirty conditions of a hatchery have a much higher risk of this.
What you describe sounds like a skin lesion which has developed into a sort of cyst or tumor over time. It could have began as a small wound or may have been just a skin lesion. Unless I began to see many trout with examples of this same type affliction I wouldn't panic.
As for deciding when the fish should die........ that would be a judgment to be made by you. If it were obviously suffering then killing it would seem appropriate. However, keeping it would not be an option for me.