I think Kelly went to the hotel room with the intention of kicking Melissa's ass because Craig stayed that night. I think Melissa may have got a little cocky with her or said something Kelly didn't like, Kelly went to her car got some tape and what ever she was bound in. Went back it, beat the hell out of Melissa, bound her, tried to scare her into god only knows, gagged her, got more pissed because Melissa couldn't respond as she was gagged and then beat her some more and choked her to death. All on a drug induced rage. Just my thoughts on it.
Here's my take....
Craig & Kelly have said they found her & she had OD.
I will take a bet that maybe it was partially true.
I bet Kelly went to go confront her about the Identity theft & Melissa was probably coherent but pretty messed.
Kelly did some yelling & screaming and Melissa told her to "fuck off" (you know how it is when your really messed & someone's yelling at you).
Kelly took advantage of the situation & prolly thought she could take her because of her condition.
Being that Melissa was really messed....she couldn't fight back very well & was killed in the process.
Kelly prolly freaked & realized she killed Melissa.
She prolly called Craig & told him what happened. At that point Craig put together a half-assed plan about trying to frame Melissa for Identity theft & stealing of the car. He went to the garage, picked up some items, and made a phone call to a friend to meet him. He then went to the Hotel & told Kelly to run to the store & meet him with some barbecue fluid. He Polly heard from a friend that barbecue fluid will generate a higher BTU of fire for incineration. He then staged an abduction type scenario where he bounded & taped her up. From there....he figured a theft of his car could back his story about Melissa being troubled & unpredictable and messing with some bad people.