Perhaps he fancied some mere chance might betray his theft if the person despoiled was not got rid of.
Perhaps to a nature in some sort primitive, almost uncivilized, and whose owner up to that time had never done anything illegal, the presence of Ursula awakened remorse.

Possibly this remorse goaded him the more because he had received his share of the property legitimately acquired.

In his own mind he no doubt attributed these stirrings of his conscience to the fact of Ursula's presence, imagining that if she were removed all his uncomfortable feelings would disappear with her.

But still, after all, perhaps crime has its own doctrine of perfection.

A beginning of evil demands its end; a first stab must be followed by the blow that kills.