I've decided that I'm going to be mean to my cousin.
Is there something about your cousin that makes you want to be mean to them?
He never does his studies, which makes him a loser. People like him should die.
While studying and working hard is important when it comes to succeed in life, that doesn't mean you should treat them hatefully and it definitely doesn't mean you  should want them to die. You should try to help them and be supportive.
He makes me mad. He should just study hard since he wants a good job.
While he should focus more on his studies, it isn't good for you to be mean to him. He's your family member and you should try to build him up rather than break him down.