Not a big leap.
Time. She has now seen evidence that Caruso had her DNA, and she knows there’s only one reason he would need that. She also knows he’s being secretive with his intentions for the virus; it’s not a stretch to think she would conclude he had perfected the virus to work well enough to try to take her out as his first target. Her only hope would be to stop him before he could. Even if she left the grounds and waited for Ether to deal with Caruso themselves, she would still be at risk if he released the virus into the world, as it would seek her out even after the fact. The idea would be that in imminent fear for her life, she took him out before he could act, and it just turned out she was too late. And it’s not like she could go to Ether with it, she just burned her evidence, and that’s not even my doing, she actually does that in-game, despite it being what Ether would need to conclude Caruso shouldn’t be part of the project anymore. So she’s clearly planning to take a different option in the eyes of whoever learns that.
Statistically, yes, but poison takes too long and is uncertain when someone will consume it. 47’s an expert, he can do it easily - right moment, right amount - but it wouldn’t be reliable enough for her purposes. More pragmatically, because using the knife lets you put it in her room to look like she set it there with the intent to dispose of it later but died before she could.
Ether put DeSantis on the project to begin with for the express purpose of replacing Caruso; they already strongly suspected it was gonna have to happen, so they’re already prepared to cover up anything unfortunate that happens to him. Add in the fact that Providence directly starts investigating it and you know they’d be able to sideline the normal investigators.
Doesn’t have to be. DeSantis knows they’re building a weapon that kills based on DNA, knows the guy who built it hates her, and now knows he had her DNA. That’s all the motive she needed. Remember, they are both dying in this scenario; the idea is that DeSantis killed Caruso and had a plan to get away with it, but died from the virus before she could, which is why their deaths were found out about in the first place. The whole reason there’s an investigation of that magnitude is that they both died and all the evidence I’ve carefully left will indicate that they killed each other, DeSantis out of fear and Caruso out of hate, and anything that doesn’t add up to what DeSantis thought she was going to do to get away with it can be chalked up to her dying before she could do whatever she was planning next.
And in the end, it’s really just Providence that needs to be deceived by all of this, at least long enough to not suspect anything beyond two coworkers killing each other over personal dislike, until the Constant opens that vault in New York.
At the risk of kicking an anthill, anything to say about the other two scenarios?