Why be nice when you can can get the same by being cruel

[Tellos Athenaios]

In the most simple theoretical cost/benefit terms, cruelty works only if you are undisputed and it is a cause for others to challenge you. There's a long term hidden cost.
Kindness just works and is a cause for others to support you. There's a long term interest.

Or: if you are a member of a social species you rely on other members not to hurt you more than they help you. If they breach that unspoken agreement it is time for being vicious.