I feel bad because I bought animals from Petco?

Well, 9 years ago I got my first guinea pig from a little old local petshop. Now, I wished we'd gotten her brother, but that's not important. She's healthy, hardy, and seems to have a life-time contract! About 5 or 6 years ago we felt she was too lonely so we got her a friend. We had to go to Petco because it was the only place avaliable around where I live that sold another female guinea pig. She was a mess...always sick and in fact the said a vet had checked her that afternoon and gave her a clean bill of health. She was sick, it was obvious. One of the girls that worked there was fond of the little girlie and told us the store was going to put her down if no one bought her today (this was after we bought her) because they didn't want to risk infection of the other animals...at the time I was thinking "Okay, well they are just trying to prevent illness" but now I'm thinking "What the **** is wrong with them?" I didn't know about all these bad things Petco had down (the articles about them buying all those poor cruelly treated animals and then backfiring for MONEY!!!)!! 6 months ago we got two more...why? I dont know! My sister persisted that petco doesnt do anything wrong! In the cage there were two other sick guinea pigs (there were obvious signs) but we got two anyways and a different girl said "Aww I'm gonna miss these guys, they were the good ones" like....umm...excuse me? What in the world is that supposed 2 mean? The manager also assured us all animals are checked by the vet and given a clean bill of health, when we asked about the others. I wanted to get the others, for the sake that I couldn't leave the poor things, but unfortunately didnt. I no longer support PetCo in buying live animals and refuse to support them whatsoever. But my friends only support shelter animals, as I always have (I volunteer) but when they ask where I got my animals I always dodge the questions because I don't want them to know that I supported Petco. I feel so bad that I bought them, but sort of good because some people buy them to feed them to their snakes or they could've just been put down in the back of the store. I'm torn between buying the animals from PetCo because I worry about them being mis-treated but I don't want to be a supporter....Should I really feel bad for this..indirect-ish support??? Plz help..thanks ;/