A scientist once confided in me that he became a scientist because as a child he really liked lizards, but as a scientist, he spends much of his time murdering lizards. :-/

Everyone involved has to confront this reality on their own, come to terms with it, and figure out the line where they're willing to meet it. All the researchers I've known have cared deeply about the welfare of the animals, despite sometimes doing terrible things to them for science. They worked to limit their suffering and dispatch them as humanely as possible. Many rationalize it by comparing to the food system, which raises and slaughters orders of magnitude more souls, and keeps people living, but does not discover or record as much new knowledge as science.