Are these sentences correct

Hello.
Help me, please, with the following sentences. Do they sound English?
1. I believe that scientists act badly in relation to animals.
2. You can't even imagine how many mice died to check one of the scientists' guesses.
3. Of course there are people who are indifferent (to) what (how) animals feel.
4. I don't agree that animals don't understand anything. (I disagree that animals don't understand anything)
Are there commas after "quite the opposite" and "for example"?
Can I interchangenably use the following words: perform/conduct/carry out an experiment; animals/creatures/living beings; experiments/tests?
Thank you!