The Suicider (canon name) is an enemy first encountered in the city and the only common foe to be witnessed using a firearm. This creature will charge the player with a Glock 19, firing rapidly with no care for self-preservation, though after the Standalone build's AI updates they will occasionally stand at a distance, shooting at the player with deceptively keen accuracy and not getting closer. They are evidently a perfect means of acquiring large amounts of ammunition for the Glock. The player can either choose to face the creature in a gun battle in which headshots are extremely effective and easy to pull off -- due to its big head -- or melee it, which is riskier.
Once the Suicider has emptied its magazine or the player has gotten too close, it will stop and raise the Glock to its own head, killing itself. This is likely meant to be symbolic of Simon's desire to do the same to himself in the real-world, the theme of suicide being alluded to by this less-than-subtle creature. Also of note is that if the player does not have a glock in his inventory and picks one up from a Suicider that has killed itself, there will still be a bullet in the chamber. Though it will still provide a full magazine if the player has a glock. If one looks closely the Suicider wears the same clothes as the Psycho. The last bullet in the chamber when Simon picks up a Glock when he does not have one is a gift for Simon to kill himself with.