At first, it seems unbelievable that Wesker missed hitting Rebecca, even though she was right in front of him and only 5ft. away. That is, until you remember Chris' crack at the beginning about Wesker's sunglasses:
Richard's few remaining hours are spent trapped in a mansion with even worse dangers than the ones outside. And just when it seems help finally arrives, his teammates keep abandoning him; including their only medic. The situation must've seemed hopeless to him, so it's no wonder he finally snapped and tried to take it out on Chris.
Despite its humor, the lyrics of "Your Brains" are pretty horrifyng when really think about it; verse 2 in particular:
Bob: I don't want to nitpick Tom, but is this really your plan?
Spend your whole life locked inside a mall?
Maybe that's okay for now, but someday you'll be out of food and guns