How did the actions of British and Hessian troops sometimes undermine Howe's efforts to defeat Washington?
Hessians were professional mercenaries, soldiers from Prussia who had been in the military for years, were seasoned battle veterans and who, like the British forces, had little respect for the colonials they had fought with and defeated so often by that point in the war. This means they also had little respect for civilians or civilian property, which they often helped themselves to as they passed through towns, especially those they considered to be sympathetic to the rebels.
Many of the British troops were draftees, and more than a little irked that they had been sent here at all. They felt the colonials had it pretty good, the loyalists understood this and the rebels were simply criminals. This was also in line with a general disdain that people from mainland Britain and high British society felt for the colonial "Americans".
Howe would have more difficulty defeating Washington and successfully pacifying territory and towns if the population was hostile, and the actions of the Hessians and many of his own troops simply made that more difficult.