Is violence/killing just a waste of time ?

It doesn't solve or change anything. All it does is make things worse. The past is the past. Moving on solves problems. 

not only that cause it just make the world worse continue cycle of hatred and vengeance that never ends. someone has to break the cycle be strong and forgive and care about everyone.

The problem with violence/killing someone it that it usually makes us look worse we have to repay a debt to society far larger than the person that deserved being killed

No. Violence & killing really doesn’t solve anything. You can’t solve hate with more hate.