Well, let's look at Danny for a moment. He got his -leg- caught in a bear trap. Looking back at the band teacher in the woods, I don't believe that's a pleasant experience. Now, it's likely that, unlike the teacher's predicament, this trap has a release mechanism. That's a fine howdy-do, but that's still a grievous wound. This is a trap meant to immobilize -bears- stuck to a human. Even if he gets free, that wound will be constantly bleeding while he tries to get somewhere to patch it up. That barn didn't have anything useful in that sense. The only place he can go is the house. Not to mention that Danny doesn't strike me as the brains of the operation. I'm not convinced he could treat the wound properly. Danny would die of infection or even bleeding out, unless the walkers got to him first.
Brenda was doomed from the start, an unfortunate victim of circumstance. She got what she deserved, but there was nothing that could have been done. Karma's a bitch.
Andy was the only one that you had much direct control over. And judging by the options when speaking to him, at least Lee is convinced that Andy's family is dead. With this news, Andy is completely broken, having lost everything. There's no greater punishment in existence. In fact, killing him would only serve to make things easier for him.
I will agree with you. In this situation, killing them is certainly the lesser of two evils. But to a child, where morals are often a case of black or white, right or wrong, you did the right thing by not giving in to your rage.