To be honest Captain Boomerang is a guy who really deserves to be tortured, at least in the comics, I'm just saying. His entire schtick is an arrogant, racist, sleazebag. There aren't too many things that are too bad to happen to him.
I'll also say that they really do need to keep "Arrow" dark and brutal to be honest, and keep to an "the end justifies the means" attitude. The reason is simply that if they lighten everything up then there won't be any contrast between the two shows, and the way they have defined things "Arrow" lacks the high camp touches that allows the comic book version to get away with the attitudes he does. See, it's easy to be a moralist when your super powerful, and in the case of "The Flash" are going to recovery from injuries and such in record time. When your just a normal guy things are a lot different, especially when your fighting guys much more powerful than you are. In the comics where gadgets are camped up to the point of entirely ignoring physics and common sense it blurs that distinction, in the comics Green Arrow pretty much pulls out the "do anything" arrow to solve problems, including some crazy Rube Goldberg seeming arrows with things like boxing gloves on the end (I think they even made references about this in the show). In the TV show he's pretty much got normal arrows, and what gadgets he has are borderline believable in a modern context, he's not going to say both freeze someone in a block of nice, and then later say unfreeze Superman with his special "de-icing arrow" he carries just for that purpose.
Ideally you'd probably see a Batman-Superman type relationship here, Batman doesn't kill (that's part of the quintessential code) but he does torture people and so on, albeit Batman doesn't pull out the hooks and razor blades so much as he just beats on people until they talk, dangles them off buildings (or sticks their head through a manhole cover into speeding traffic), or heck even drops them from great heights and catches them at the last second with a batarang. Sure Batman doesn't kill, but most criminals who run into him wind up being REALLY unsure of that unless like The Joker and other key rogues gallery members they have dealt with him enough to put it to the test.
My attitude might be different if we had an actual Batman show, but right now we've pretty much got Arrow and The Flash and to me having one of them remain a "Dark Hero" makes sense as does playing styles off each other while maintaining a degree of respect for each other's methods even where they disagree.
What might be interesting as a storyline for another crossover might be for The Flash to guilt Arrow into trying to play things totally "straight" and get pulverized for it and put in a coma (he doesn't heal super fast) while he'll of course recover The Flash getting more respect for his methods and why a normal guy needs to act like that to be a super hero might come out of it. It would give an excuse to have The Flash step up in power level as he tries to protect both cities in Arrow's absence.
Just my thoughts at any rate. Too much "dark" ruins comic books especially when you don't have anyone to contrast those heroes with when everyone becomes a dark hero. With the TV shows however doing one each way makes sense.
