He spared the old man and even give him a purpose.
Train his knife strike and one day, come again to take revenge on him
For a man who loses everything what Loki should do is give him a purpose.
At least he would try to live. And who knows? Maybe, someday, he would find new joy in life.
Then Loki has the merit of saving a life.
In the kitchen, the old man is making coffee but Loki is not interested in knowing whether the old man put poison or not in his drinks.
Instead, Loki wanted to relax for a second.
The journey toward the Untainted Island has not been kind to him.
And it is also because there is injuries in his neck.
He did not show it but that old man strike did hurt him. It is just that he managed to turn the truth to lies just at the right moment
that knife strike almost reached the level of Concepts and touches upon the Law of Destruction.
It is a strike that old man has been perfecting all of his life.