BoVD gives as an example- If you are fleeing monsters more powerful than you- and in order to escape, you must cross a dangerous mountainside that is highly likely to landslip and kill people, and you do it anyway (because you value your life more than theirs)- that's Evil.
"Sacrificing others to save yourself is an evil act"
As to the whole issue of deterrence- with not torturing people horribly being called an evil act because it leads to higher crime rate- imagine if alignment did work that way:
Paladin ruler signs a decree replacing crucifixion with swift, relatively painless, beheadings for really serious crimes which are evil acts.
DM: "20 more murders will happen because you reduced deterrence. You caused those 20 murders. You Fall."
Paladin player: "Right! No more merciful beheadings!!"
Doesn't sound very fair- or very consistant with D&D.
Prohibitions on "cruel and unusual punishment" go back some way- and D&D morality tends to be based on more recent principles, such as that one.