It's only human nature for Shafer to badly want to beat SU. I'm sure he feels like we pulled the plug too early on his tenure, and that there were circumstances beyond his control that negatively impacted our performance (Hunt's injury, Marrone's disinterest in recruiting, etc).
It's also possible for fans to have nuanced thoughts about him. I think the world of Shafer on a personal level, and I wish he had more success at SU. If he had won I think his personality would have lent itself to getting Mac-like love from the fanbase. But he wasn't a good head coach, it was clear he didn't have the chops to turn the program around, and I want SU to kick MTSU's ass six ways to Sunday. Nothing personal, just business.
Last thought... I find the notion that he has some advantage because he "knows our personnel" to be laughable. We don't run anything resembling his offensive and defensive styles. Just because he recruited some of our roster doesn't mean he has a better idea than any other DC on how to defend what Dino is having them do.