By design
It is very important to remember that many veterans are hostile to Kerry not because of any alleged weird stuff he pulled on the river, but because of his well-documented actions after he returned from the war. This isn't being split off properly in this controversy.
Well, that's by design. I assume you've seen the ad by now. The statements about Kerry having betrayed his comrades and his country seem to be about his opposition to the war once he returned home, but they're edited and spliced in such a way as to remove the context and make it sound as if they're charging him with criminal misdeeds while still serving in Vietnam. Vicious, but hardly unprecedented.