Character Analysis
What can we say about Vasili Nikitin that hasn't already been said about infectious diseases? The is one no-good dude, obsessed with power and willing to do whatever it takes to gain it. In fact, his hatred of Leo is rooted in how differently the two men think.
Now, let's be real: Vasili is a pretty one-dimensional guy. Yeah, we said it—he has one less dimension than Pac-Man. This chump will do anything if it means that he'll move up a rung on the ladder; he'll even rat on his brother for "anti-Stalinist remarks" muttered while "he'd been drunk [...] celebrating his birthday" (2.5.38). It doesn't get a lot lower than that.
Vasili hates Leo because Leo reminds him of what an awful dude he really is. Leo doesn't crave power; he wants to help people. Leo wouldn't rat out his family—in fact, he stands up for his wife, even though he knows it could lead to his death. With Leo around, Vasili is constantly "aware of his actions as pathetic"—though it turns out that trying to get Leo killed doesn't seem to make him feel any better (4.45.14).
In the end, however, Vasili isn't under any illusions—he knows exactly who and what he is. Every hero needs a villain, and Vasili is more than happy to fill the position.