There are several reasons to seek damages. For one, that man’s family deserves some kind of compensation for the way they were wronged. For another, punitive damages might help prompt the Sheriff’s department* to change their tactics in the future. This case wasn’t a matter of one murderous cop going rogue, it’s a matter of several cops acting on standing policy.
*Yes, it’s ultimately the taxpayer’s money. But taxpayers and government officials are more likely to demand change and fire some bad cops when they get hit in the pocketbook.