Jealousy can easily set a man to a despicable state.
In an organisation, each person can have different job scopes; and some might have a lighter load for a lesser pay. Some work is non-visible and thus, others would probably misjudge.
Nevertheless, it is extremely insensible and hypocritical to backstab your co-worker for not aiding you when boss has already clarified the workload, whether you have to cover all the shit or not.
If your co-worker has the time and capability, what do you think he will react based on your judgement on him?
Have you ever boosted about how good you can manage your work but secretly kick a big fuss out of a co-worker who does not aid you?
Unfortunately, human beings often do not realise what they are doing may be wrong.