I went to Silver Rush and Jean-Baptiste asked me to go get Cass. He didn't say why, so I thought, "Um, ok".

I talk to Cass but as I recall she wasn't saying anything too helpful and wouldn't come with me. I remembered her old quest to go talk to Crimson Caravans, so I did.

Crimson Caravans told me to go buy out Cass' business so I go back to Cass. I do the speech challenge for 50 and get her to sell, then come with me.

I immediately go to Silver Rush and talk to Jean again. It's clear now that he's up to no good, so when he makes a move I defend Cass and kill everyone in the shop. (Reinforced Combat Armor is king against lasers with low damage)

Now I continue on Cass' quest and go find all the attacked caravans. She's pissed and wants to go kill Alice McLafferty of Crimson Caravans. I ask if I can find evidence instead, and she said, "Fine".

I go to Crimson Caravans and find the evidence. Cass immediately talks to me and is seeing things my way now.

I go back to Silver Rush and find the evidence there. I talk to Cass and now things have changed. She's all, "You wanted evidence and yet you kill everyone." I did at Silver Rush a loooong time ago. And now Cass has given up on the evidence route and now wants to do things her way and kill Alice again.

WTF? She knew I had already killed everyone when she agreed to try the evidence route. She shouldn't give up on that route when I killed everyone at Silver Rush before I even knew there was an evidence route. So now I have to go kill Alice and piss off the NCR who I'm friendly with, or just abandon the quest, when I have both pieces of evidence! or I can load a 3 hour old save before I killed everyone at Silver Rush. The things Cass said to me make sense if I killed the Van Graffs after trying to collect evidence, but not before when trying to defend Cass.

I took Cass to Jean-Baptiste, tried to protect her, but she got herself wrecked, so I reloaded and just didn't do that. The whole thing worked fine with everyone alive, so what should really happen is that you take the evidence to Jackson at the end and he says there's nothing he can do with the Van Graffs dead. Then Cass would say the only thing left is to kill McLafferty. At that point you're still stuck in terms of having to complete the quest by offing the main caravan runner in New Vegas, but it's logical.

In real life, if there are two people who conspire to commit a murder and you have evidence proving this, just because one of the conspirators is dead doesn't mean the other one can't be prosecuted. I think Ranger Jackson should still accept the evidence against Crimson Caravans, thus resulting in the "good" conclusion to the quest. You only killed the Van Graffs out of self defense and protection of Cass, you don't deserve the to be forced to take the evil path.