This Shloka is for the benefit and best interest of mankind. Saving oneself from harm and calamity is the best and greatest duty so the Lord has stipulated that one must act in such stuations to save themselves.
Mokshadharma explains, ‘One’s Dharma during easy (favourable) times is different to that of hard (unfavourable) times.’ For such reason, one may even adopt another’s Dharma, untruth or even break the code of untouchability (i.e. a Sadhu touching a woman) in order to save oneself or others from harm.’
A ‘calamity’ here should be understood as something that is the cause of great pain or misery. Thus, some great difficulty that has hit a village or town, the wealth of the kingdom has been looted or some great illness has struck all constitute a calamity. Similarly if one is personally suppressed or threatened with violence then one should act in such a way as to save oneself and others from such tyranny.