Help with Questions to All Quiet On The Western Front : As Paul stands guard over the Russian prisoners

As Paul stands guard over the Russian prisoners, he ponders how commands from higher-ups have transformed men so like his own countrymen into enemies and could just as swiftly turn them into friends. But his thoughts frighten him. What is “the abyss” to which he fears such thoughts will lead?

Why does Paul feel a “strange attachment” to the soldiers in his outfit once he returns from leave?