Just prior to the dance Col Brandon visits Elinor and hearing of the regard that is apparent between Willoughby and Marianne wishes her all the happiness and that Willoughby "...endeavour to deserve her."

Well, he knows something of Willoughby's true character as we see in the incident at the dance. Poor Marianne! To have your heart broken by someone you love and look up to is one thing. To have it done is so cruel and heartless a manner just adds insult to injury.

If Willoughby had indeed tired of Marianne or whatever, the fact that he would leave her hanging without any word and then unceremoniously ignore her at a public gathering while she so openly and affectionately addresses him shows just how selfish a person he truly is.

To use a modern word, he acted like a true wimp. He knew he'd likely run into her or have to face her at some point. Why leave all to chance in this way?