I'm going to address the dog analogy too. If you bring a dog into a situation where he doesn't fit, it is really your own fault for not checking out the dog first. You should never have allowed Rex to play with Fido and the rest if you didn't watch them a little first to see if they would get along. It is the responsibility of the owner not to put their dog into the situation where it will get into that kind of trouble. If someone else bought you Rex because he was just so darn cute, and you knew or suspected he wouldn't get along with your other dogs, then you tell them "no thanks, please take him back and ask before you get me another dog." This is exactly the same as with fish... it is the owner's responsibility to prevent any compatibility issues (not deal with them once they've already happened.
The honey wouldn't have been a problem if you thought before you threw him in with other, smaller gouramis (Gouramis are related to BETTAS, people). He probably would have been fine in the big tank, once the other gouramis were gone. Regardless, killing him doesn't bring back the molly or the other gouramis. And if the QT tank is for QTing, then what was the molly for?! And if you just got the molly, how do you know it didn't just die because it started sick? You obviously couldn't have QTed it first.
I see no reason not to take the fish back to the store rather than killing it. Someone else could put him in a perfectly acceptable home. Same with Rex... he'd probably do fine in a single-dog home.
And at the risk of sounding hypocritical, I see no problem with what lordroad did. He likely saved the angel's life by removing the gourami, and he had nowhere else to put it, and it was likely spreading disease. Your fish was healthy and not causing any problems when you decided to kill it, and you could have just as easily taken him back with no risk to your other fish if you had to wait.