Wait, why was Sparky not as evil as the other animals?

There wasn't a Charlie in the film,

that still made absolutely no sense. What does love have anything to do with science?

I'll look down, and whisper no

I wondered that, too. I don't think that all of them would have eventually vanished. There was a sort of sliding scale. Victor's dog, of course, came back exactly as he was. Victor was the only one with 100% pure motives. It was clear that Nassor truly loved Colossus, and while that little guy didn't come back the same hamster (at least, I don't think he did! xD), he wasn't as powerful a threat as the others. Toshiaki likewise loved Shelley--but he only expressed his caring after the turtle had been "re-killed," finally dropping his camcorder and appearing sorrowful instead of devious. Shelley was quite a monster, however, without the Miracle-Gro accident, she probably wouldn't have been quite as bad. On the other hand, those remaining creatures...the rat Edgar pulled out of a trash can, the random dead fish he picked from the pet store, the sea monkeys that were only going to serve as part of a "lame" science experiment anyway, the deceased bat fetched by Mr. Whiskers...there was no emotional connection at all between these creatures and the children bringing them back. Therefore, they were bound to come out "messed up." Would they have eventually disappeared/vaporized? One might have to ask Tim Burton, but I don't necessarily believe so. At least with them, the kids had science fair glory as a strong motivating factor. In the case of the fish, it was merely something Victor wanted to do quickly to appease Edgar's curiosity. The poor fish stood no chance of coming back to life the right way--and surely would have been the quickest to return to the afterlife, at any rate. :/

And as for love not having anything to do with science...okay, a lack of passionate love for an experiment won't literally, in our world, lead you to create a rampaging, citizen-terrorizing, town-destroying beast. But even in something as seemingly cold, sterile, objective, and calculating as the sciences, it IS important to feel a real love for your pursuit of knowledge and what you are doing. As Mr. Rzykruski said, science itself is not good or evil; it can, however, be used in both ways. The idea that taking what scientific knowledge can do for us and using it in unscrupulous, unwise ways can lead to dangerous, negative, evil consequences is VERY applicable to the real world. A scientist's work ought to be motivated by love.