You might not feel negative effects right away, if it was just once or if it was self defence, but if you wake up one morning and go 'I'm bored, I think I'll curse that person' you will most likely get negative energy back in your direction. You might not get beaten up, but you might get a pop quiz in a class you are doing poorly in, forget your homework on the bus, or feel bogged down by the energy. Some even believe your next life you will get rewards or punishments depending on this one. Like a murderer will be a victim in the next life [I don't believe this, but some do] basically do your best and the universe has a way of working things out.
Regarding death spells you are casting a very powerful curse which would require a lot of energy to pull off. Choosing to be judge, jury, and executioner can bring very negative effects because humans should not possess this power. Doesn't matter if this person hurt you, look at the big picture.