To be fair, that doesn't happen a lot in TOS. Running towards danger is chiefly a TNG S1 phenomenon, before the "no running with phasers" rule was imposed by Chief Grandma.
TOS guards see fairly little shipboard action overall. In "The Changeling", they operate in pairs on what rightly amounts to a suicide mission. In "Doomsday Machine", a single guard puts up a good fight against Decker, who benefits from his Starship Captain superpowers in a rare piece of beyond-main-cast continuity (Tracey later portrays the same superpowers) in a situation where the single redshirt probably would have triumphed over anybody else. Elsewhere, the guards just stand with their backs against the wall (not the door), which is a problem if the door isn't locked. But it's probably supposed to be, again barring superpowers like Khan's.
Nemesis can be excused for its failing internal security on two main grounds. As the ship is battle-damaged, the intruders could be exploiting gaps in internal defenses - and for all we know, Security was blocking those gaps, having its hands full, this leading to the final confrontation where the Viceroy's whittled-down force of survivors was facing the Deck 9 last stand of the A Team of Security.
Timo Saloniemi