Not infantile at all. It's trying to become more moral by not stooping to common methods, which killing is considered. Killing is considered base and feeding in to humanity's lesser nature, while not killing them is "taking the higher road." And it's a trope repeated throughout Star Trek so it's one that definitely I see around here a lot.
Do I agree with it? No. Sometimes ending the threat is the best possible path. Sometimes revenge works out for the person doing so.
It's not infantile. Infantile is very much "eye for eye." Watch youngers and see how they deal with being wronged. Kid pushes one and takes the toy and the wronged person immediately gets up and goes to fight them. Seen it multiple times working with child clients and youth.
I don't think it's wrong to want revenge. I just think that sometimes it's not the best way.
I should add that the character of Teal'c in Stargate gets a similar arc.
Frank Moses in RED gets this as well.
Travolta's character in "Face Off" gets this.
I think it happens more than we realize. Just not quite as iconic as Princess Bride, for a number of reasons.