Deranged, or perhaps even panicking, considering that Grell being the attacker has been brought up as a viable option. Without knowing the motives and the feelings of the actual killer, however, we can't say for sure either way. And you're right - to narrow down culprits like that, without evidence, would be falling right back into what we did last week.

But regardless of whether they attacked her, or were defending themselves, further evidence on the body would be my only guess as to why they'd try to incinerate her body. Or thinking that they could hide it. [ That would be two missing bodies... you can see the headache behind his eyes at the thought. ]