I doubt that's how it really went down, but the situation being what it is, I see no need to question it.

My guess the guy heard him come in and waited at the top of the stairs and if he said anything at all it was "So long, Sucker!!!" and then ran downstairs to make sure he didn't have any accomplices or get-away drivers that needed silencing. Which is exactly what I'd have done.

But like I said, the ends justify the means.