General lack of care:
* Apathetic
* Indifferent
* Cold
* Callous
* Heartless
* Unfeeling
* Aloof
* Detached
More specific connotations:
* Selfish: Focused solely on one's own needs, without regard for others.
* Insensitive: Lacking awareness or understanding of others' feelings.
* Thoughtless: Acting without considering the consequences for others.
* Neglectful: Failing to provide necessary care or attention.
* Cruel: Intentionally causing pain or suffering.
* Ruthless: Showing no mercy or compassion.
* Unconcerned: Not worried or bothered about something.
The best synonym will depend on the context of your sentence.