I agree Mueller had it in for Trump, evidenced by what he did to Trump associates and the fact he packed his team with Democrats, and so on.
Why he didn’t fabricate the report? My guess his hatred for Trump didn’t trump Mueller’s sense of self preservation? Trump over 2 years didn’t fold. So Mueller was on the verge of a coup de tat and chickened out? Trump’s base was solidly behind him and in fact Trump even gained some never Trumpers on the right, like Graham and Cruz, to his side. There is also the chance that among the public in general, Trump is more popular than the polls suggest. Mueller would have been standing before the American public looking like Mike Myers with a head in his fist.