Why were the models being killed?

Exactly right. And with the models dead they wouldn't have to pay them.

It's kind of a plot hole, honestly. The reason is, if not stated outright, at least heavily implied, that Digital Matrix, Inc. would have to pay the models for the use of their likenesses, even after it switched over to using computer-generated avatars of them in its commercials. They tried surgically modifying the girls so that they were "perfect" but the problem was the girls couldn't act, move, speak, etc. enough to maintain that "perfection" so the company went with computer-generated versions that could. Once this was done, the girls themselves were no longer needed. John Reston, who owns the company, does not want to have to keep paying them now that they are surplus to requirements, so he has them killed so that he doesn't have to. Once they are dead, he owns their likenesses free and clear, and he can use his stable of computer-generated models for these subliminal message commercials, over and over again at no additional cost.

I say it's a plot hole because in reality, it would be far, far smarter for the owner of a huge corporation, worth billions of dollars, to drop a couple hundred thousand dollars each on these girls' and sign them to a contract surrendering all rights to their digital likenesses in perpetuity, than it would be to commit multiple premeditated murders, drawing the attention of law enforcement, and risking life sentences, or even the death penalty. But... if Reston hadn't decided to have the girls killed, there would be no movie, so there you are.