Points are used for upgrades, such as increased ammo capacity, some weapon attachments, and such. One of the most useful investments being the upgrade that gives you health from kills, however illogical that is.
Killing perps with torture is worth -20 points and obviously no rewards for successful interrogation or the subsequent Quick Kill.
Torturing wounded enemies is easier as they regain their composure much slower and feel more threatened by it, but as some methods actively harm them, this may result in their untimely death.
Successful interrogation restores a small portion of your health. This is also the only way for recovery besides the upgrade that gives you health from kills.