Once the monster's focus has been established it will always move to attack it's focus. If the ONLY path to the focus is a trap, it will move onto the trap. Maximizing its attack by attacking a second target is secondary.
That isn't entirely true. The trap is factored into determing the focus.
If there is a trap-free path to a hex from which the monster can attack ANY enemy (regardless of path length), it will not step onto a trap (or hazard). Its focus would be the enemy which it can reach (trap-free) with the least amount of movement.