"ruthless behavior" is a correct and usable phrase in written English.
You can use it to describe someone who is acting without any empathy or conscience. For example, "The CEO's ruthless behavior in laying off all of the workers without giving them severance pay was questioned by many people in the community."
The speed of Phones 4u's decline led its founder, John Caudwell, to accuse the networks of "astonishingly ruthless" behaviour.
No wonder the agent who represents many of the country's leading stars says privately that Footballers Wivess, a satire on the dubious morals and ostentatious 'bling' lifestyles of today's millionaire players, does not do justice to the sly, slimy, dishonest and ruthless behaviour of some figures in the game.
In the italicised sections where he gets to express his own views on any given topic, the singer often laments his own capacity for cold, ruthless behaviour towards others - particularly co-workers - and readily admits he's a self-absorbed, generally anti-social person.
Dressed in post-punk Vivienne Westwood, all angled lines and dominatrix boots, her Alcina was not so much a woman trapped and destroyed by her own emotions as an arch-manipulator, whose power over others wanes as her ruthless behaviour is exposed.
Wormald suggests that this ruthless behaviour may be explained by the fact that landed estates were becoming harder to find, as so much land had been granted to monasteries.
We ought to be ruthless about searching out improper behaviour".
It was ruthless behavior.
So, yeah, pretty ruthless.
We must be ruthless in purging such behaviour from its ranks".
Hovering over them all is the controlling figure of the late Murry Wilson, their one-time manager, whose ruthless ambition and abusive behaviour left its mark on all three of his sons.
Describing Salmon's behaviour as coldblooded and ruthless, the judge, Johannah Cutts QC, said she had to follow sentencing guidelines in place when he carried out the offences.
