I am often on the side of untargeted hit which would put me on your side. But, I do like to play devil's advocate. So here goes.
1. MB never invited her campers to come early that morning. Her post had the "If it's raining we're still training" heading that she had used in the past as needed, she listed the time as 5 a.m.
2. As for timeline, it fits fine. He broke in at least 30 mins, maybe more, before she could reasonably be expected to arrive and another 40 mins before class would start. I'm not sure that he could have expected a camper would come 25 mins early and sit in their car, and that did nearly get him caught.
3. I don't know that it's been established what MB could actually see from her vehicle when she pulled in. And she sure isn't here to tell us. The unshared video won't tell us what she saw, either.
4. The highway isn't all that busy at 4:20 in the morning. It's dark and it's raining and the action isn't taking place outside anyway. The cameras are a non-factor because of the disguise. He kills her (in the targeted scenario) either because he intended to do so all along, or because she fights back viciously and it takes a lethal blow to subdue her, or because she recognizes him, or because he panics when the first camper pulls in unexpectedly.