Why did the Companions not stop praying to defend 'Umar ibn al-Khattab when he was stabbed while leading the prayer, and does that indicate the detestability of leaving the prayer?

It is permissible to interrupt the prayer for legitimate reasons, such as killing a snake or a scorpion, helping someone in distress, or alerting a sleeping or heedless person to danger. This is allowed if the benefit resulting from the interruption outweighs the harm of interrupting the prayer. An example of this is when the Companions left the prayer to defend the Commander of the Faithful, Umar ibn al-Khattab, may Allah be pleased with him, when he was stabbed. A group of them went out to apprehend the aggressor, while the rest continued their prayer, because the situation did not require everyone to leave, and many of them did not know what had happened until after the prayer had concluded.