The protocols for lab animal euthanasia are incredibly humane. I don't mean to criticize you, but I think perspective can help anyone come to terms with this.
When you think about what nature itself does to capture energy, that's at times incredibly savage and brutal. Gazelle on the Savannah being hunted and eaten alive by lions, rodents in the field being picked off by eagles, wasps laying eggs in live prey, fungi infecting the brains of animals and causing them to be eaten.
Lab research may seem macabre, but it doesn't hold a candle to nature itself. And our lab research seeks an end of human disease and suffering.
Even if we collectively decided that every human must behave in a way that doesn't kill any other life, one day our sun will burn out and all life on this planet will end.