In the Doctor Who episode Mummy on the Orient Express it's quickly established that the weakest passengers are being picked off first. The Doctor uses this to his advantage to guess who is going to be killed next.
However given the "mummy" is actually
an ancient soldier
This seems rather counter intuitive.
Typically soldiers don't target the week, they target military personnel and operations.
Why then does the mummy target the old and infirm?
This is a lone soldier, behind enemy lines, surrounded by individuals that it might not be able to identify as civilian versus enemy combatant. It is also weak itself, needing to drain power from its victims to recharge itself.
It stands to reason that the best tactic is to pick off the weaker adversaries first, gaining strength and dwindling the enemy numbers before going after the more high profile targets.
This is pure conjecture, but my take was that the Foretold was intentionally targeting the sick and wounded because those were those that it deemed the most likely to die soon anyway, thus allowing the strong and healthy (e.g. those with the greatest life expectancy) to survive.
CLARA: We were fighting that?
DOCTOR: So was he.
As a soldier, the clear implication is that it was forced to kill by the malfunctioning technology it carried. It's certainly possible that it was trying to work out how to do the absolute minimum possible damage and that means killing those who already have life-threatening conditions.
