. . .but he doesn't know where it comes from; no wonder he's confused.

Though I wouldn't use murder re: his actions toward not!Roger. He's acting to defend Fred (though possibly by way of overkill) -- Wesley has already decided by the end of the episode to interpret what he did as helping Fred; but it's worth noting that earlier on, when the cyborgs attacked him and not!Roger in the office, Wes moved quickly to protect not!Roger. I think his protective instincts are on display there, at least as much hatred of his father or wanting to do something for Fred.