Jen, I don't like the thought of animals being in pain as much as you do. However, I agree with the others on this one. Yes people should be aware of alternatives to testing on animals but what are these and how reliable are they? The answer is, things like computer models (I work on these) which to be honest are nowhere near as reliable as animal testing (which also has is downfalls).
To be honest, if you're testing something like pesticides, I would rather know they were tested on a few lab animals than being put through some toxicology software (which mistakenly gives it the go ahead) and then result in hundreds of animals and plants dying and generally causing havoc to the ecosystem.
Apart from the few sick individual, these scientists probably try not to think about what they are doing and see it as a necessity rather than enjoyment. I take my hat off to them that they are able to do it because I dont think I could.