Characters' Motivations?

There were motivations for almost all of the killings however slight they may have been.
Odds are that many of the random killings you see in the montages also had some type of motivation.
This is not to say he doesn't enjoy it. He enjoys it, and he's all too willing to kill on a whim.
He mostly cites income as the prime motivator. Being ridiculously pretentious and narcissitic there is a good chance he is simply lying to himself to make sense of his homicidal urges.
Either way he states it's money, and you can choose to take that at face value or not.

SPOILERS AHOY !!!

The crew's motivation to follow Ben parallels the spectator's motivation to watch the film.

The film says something about the involvment of the spectator (who is, in some ways, represented by the camera crew) by the act of witnessing violence : what are the implications of voluntarily witnessing violence (and actually seeking it by buying/ renting/copying such films)and its consequences to the psyche ? At what point do you say "Stop. This is wrong. These men are evil."

The crew at first only witnesses the violent acts comitted by Ben. Then they start helping him a little by zooming in on another rival hitman. Then they help him even more, to get inside the houses of a lonely elderly to rob her of her money (and eventually kill her in the process). Then they help him find and then restrain a fleeing kid. Then they help him dispose of bodies in the river. Then they actually cross the line and participate in the shooting of the other camera crew. And in the end all of them participate in the atrocious gangrape and slaughter of an inocent woman in front of her husband, whom they murder too.

(Note that each time they get more invovled in violence, a member of the crew gets shot. And violence is visited upon Ben's family and friends too. And of course, everybody dies at the end).

As for the spectator, it all starts as black comedy. Ben is funny, articulate, and quite charismatic/charming (and most victims anonymous common people with no defining characteristic or psychology, and no exposition time). You laugh during the first few murders, but as the violence escalates, you (should) start to wonder about how emotionally attached you've becomed to those protagonists. Are they not plain vicious sociopathic murderers ? At what point do I stop laughin and rooting for them ? At the strangling of the woman in the train ? The heart attack inducing frightening of an elderly woman ? The slaughtering of a whole family, kid included ? The home invasion and subsequent gang rape and muder of a couple ? At what point do you feel you've been conned by the "mise en scène" to like and root for people who are in fact vicious murderers with little redeeming qualities but they eloquence, and start feeling "ashamed" of having laughed in the first place, at the very first murder depicted ?

This is a really great film, and a true work of art questionning the involvment of the spectator in the act of viewing and witnessing violence, and the difficult task of trying to determine exactly where the line stands in the matter of representation of violence on the screen.

I think another very fine film which does that very well too, yet with a different style, is Cronenberg's History of Violence.