If machines could express emotions, Mac2 would be feeing pissed off. The times when Rufus 'tuned in' were increasingly fewer and when he did it was clear he was making unnecessary errors of judgement. Mac2 had no problem with the concept of killing, but it must be purposeful otherwise it added risk. Unnecessary risk also put Mac2 at risk.
Time for remedial action. When Rufus next 'tuned in' it would embed a series of brain files such that when Rufus hadn't connected for 12 hours, he would get a blinding headache that painkillers couldn't fix.
Mac2 would be his drug.