Yahtzee, apparently you know nothing about how the average gamer acts in all video games, not just GTA. If they're able to, most gamers will be completely evil psychotic bastards that would make the devil himself look good in every way that the game allows them to at some point or another, even if it's solely out of boredom. Outright coldblooded torture is a really... iffy subject so it doesn't appear that often in video games, but when it does the average player will do it for various reasons, plenty of them having the reason just being "it was fun."And if that was the intention, it bloody well worked for me, but I don't think it was, not fully. I've heard a lot of other people make the case that Trevor is intended to be a representation of the behaviour an average player shows when set loose in a sandbox game. A depiction that makes sense when he's being portrayed more as a sort of amoral free-spirited rogue, but not in the moments when he is merely nasty. A player, released in the sandbox and free of consequences, is reckless and whimsical, not cruel. They might kill someone if they're in the way or because they fly off in a hilarious manner, but prolonged and calculated torture isn't the same. It's just not as funny. The protagonist of Saint's Row IV better represents a sandbox player to my mind, because they have charisma; the satire works because they are an idealized self to match the idealized morals of the sandbox, whereas Trevor is an ugly monster.
I mean, I can't count the number of comments in a Fallout 3 Blow up Megaton video on Youtube or on this very forum who've said they did it, plenty who say they wouldn't granted, but lots of them anyway, and the same can be said for similarly evil acts in video games. The reasons for these acts vary, from "for the money/karma points" to "for the sheer fun of it" to "I was bored", but in the end it still shows that what Trevor does is pretty tame compared to the insane things the average player will do if the game allows it. GTA itself is built around feeding that psychotic nature.