I did have a similar hypothetical thought along those lines. IF the guard force had full view of what was going on from the start of the incident and IF they knew that the individual being attacked was military and their life was in immediate and mortal danger and IF they had constant PID on the attackers, where would they have been on RoE grounds regarding engaging the attackers?
The right to self defence would have meant anyone could have used "reasonable force" to prevent the loss of life. Irrelevant that the person being attacked was military. Would suggest that armed guard using a gun to prevent knife wielding nutter would be considered reasonable force.