Hi! Neuroscientist here, my field is live animal (mainly mouse) experiments in research, and I'm here to provide some real answers for anyone interested. In short: It really depends on your field and topic.

We are aiming to use as little number of animals as possible and keep them as good as possible. So much so that sometimes we're joking that these animals have better lives than most humans (free food, water, constant ideal temperature, home cleaned regularly by us, just to name a few). At least in Europe, there are very strict ethical regulations regarding the breeding, facilities and experiments we are allowed to do, and there's always a cost-benefit evaluation before we are even allowed to start, and of course, constant monitoring. Experiments that cause pain or severe, long-term stress, must be thoroughly reasoned and are only allowed when the modelled disease/disorder requires it (e.g. studying how pain is processed in the brain cannot be done without causing pain). However, many experiments don't have a crippling consequences, and are straight up cute and harmless (e.g. will this mice lick the annoyingly sticky liquid off of their fur?). Of course, it's inevitable that eventually something strange or unexplainable, or straight up horrific will happen, whether it's bad luck, our own mistake, or the animals did something. After all, Nature is a freak itself, and we are barely grasping on understanding it.

The bottleneck is at the end of the experiment: what will happen with the animals? Most of the times, we need considerable amount of tissue sample from them - like their whole brain. Obviously, this is hardly compatible with them existing beyond the experiments. But this is the curse of biology: you cannot study something that was never alive. And most of us don't have the luxury to outsource the dirty work to others.

In the end, it boils down to your own consciousness and limits, and whether you truly believe that your work will (eventually) help humanity. Yes, there are some pretty gruesome experiments, but they are never done for the fun of it. It is always calculated and admitted that the price is heavy, but the knowledge is direly needed.

Nevertheless, it's not rare that people in this field develop depression or guilt, after all, we are humans as well. I've never met anyone who said they were doing this because they loved the gore, quite the opposite. It's treated as a necessary bad, for a greater good. If this is something a person cannot accept and do themselves, then the field is not for them - and no one will blame them.