Parental alienation can become a problem when one parent either overtly or covertly does or says things intended to end the relationship between the other spouse and the children. This is probably the most destructive behavior in a divorce.
It is never in the best interest of the children to poison their minds against the other spouse. Children love both parents equally, but in the long run, children will remember the alienator's activity. When they grow old enough to understand what has happened, children may end up rejecting the parent who attempted to turn them against the other parent.
In the short term, it may appear to the alienator that this behavior is some sort of deserved punishment directed at the other parent, but ultimately this kind of behavior can be very destructive to the children.