Though most true bugs eat plant sap, some feed on animals. One such true bug is the assassin bug, which feeds on fellow insects.
They get their name from their hunting prowess. They're strong and good at overpowering victims much larger than themselves. They have raptorial forelegs similar to a praying mantis, meaning they're covered in teeth-like extensions used for grasping prey. Their rostrum, the tube they use for sucking up their meals, is curved outward from their heads in such a way that it can swing out to pierce their victims.
They suck blood other nutrients from the victim. Once an assassin bug pierces an insect, it pumps a salivary secretion into the victim's body. This secretion not only paralyzes the prey, but it dissolves the insect's insides. In effect, the secretion digests the meal before the insect sucks it up. After it dines, all that's left of the prey is a dry exoskeleton.
Assassin bugs can, and have bit humans. Assassin bug bites can be quite painful because the secretion kills off a small area of cells, but that's the extent of it really.