In normal riding, the down tube is in tension and the top tube is in compression. With a severe dent like that in the top tube and a moderately heavy rider, the top tube could simply fold at the dent under the right circumstances. The worst place for a dent like that is in the middle of the tube.

I'd walk away unless you've just got to have that frameset and are willing to spend more money to replace/paint the top tube than the frame is worth.