brutal, cold-blooded
the cold-blooded killing of a young woman

deliberate, intentional, targeted
indiscriminate, random, senseless, wanton
mass, serial, spree (AmE)
This was the fourth mass killing in Australia in four years.

contract, gangland (esp. BrE), honour/honor, political, revenge
a brutal revenge killing

mercy
Should the law allow mercy killing?

extrajudicial, unlawful (esp. BrE)
a verdict of unlawful killing

be responsible for, carry out
order
The Mafia ordered the killing.

justify
How can they justify the killing of innocent people?

prevent, stop
It is difficult to prevent such killings.

condemn
a resolution condemning the killing of Palestinian civilians

confess to

happen, occur, take place
continue
the killings that continued unchecked for days

a motive for the killing
No motive for the killing has yet been established.

make
Investors are set to make a killing from the sell-off.