Some of it is because of annoyance and need too.
here I am, trying to be a good overseer and make all my dwarves happy, when some dwarf decides that she's just TOO depressed after I temporarily took away her burial ground for renovations. She tehn goes crazy and starts attacking people. For the good of the fort, she has to die. The same goes with nobles who ask for spoiler-made mini forges and are willing to beat the crap out of people if they don't get them. They need to die for the good of the fort.
Elves have a lot lower mortality rate since they stopped bugging us about trees I think. However, when you painstakingly go through all your artifacts and accidentally let one wood-decorated bauble escape your keen eye - and then they start to leave with all their tame giant eagles... what are you supposed to do. Reason with them? Slaughtering them and taking the eagles is more satisfying.
Let's see... what else. Migrant waves ruin hours of planning. Lazy dwarves can cause the downfall of a whole fortress (we NEED shroom wine to survive yo). cats and dogs can outbreed your computers ability to track them (and provide a highly-necessary and tasty meat).
And then there are just accidents. "oh, that warrior has a baby? so? I'm a feminist and she's awesome" turns into "holy crap, she carried her baby and used it as a meat shield against the spikes! Is she crazy!?" until it happens about 30 times and you just don't care anymore. The baby had it coming.
And then there are benefits to a bit of sadistic thinking. Like the spartans, trained from birth to kill, these dwarves are hit by siege after siege after forgotten beast and titans until FUN happens. If you are harsh and kill the unnecessary dwarves/animals, then your dwarves get used to death. Then when some freaken beast tears through half your fort, the other half doesn't start throwing fits. Instead, they just become awesome.