isomori

Ну так-то тут обет можно интерпретировать как данный в рамках квеста по убийству Дамблдора.
"Will you, Severus, watch over my son as he attempts to fulfill the Dark Lord's wishes?
And will you, to the best of your ability, protect him from harm?
And, should it prove necessary... if it seems Draco will fail... will you carry out the deed that the Dark Lord has ordered Draco to perform?"

Если Драко топится, то это не по ТЗ лорда.