Do all 4 madhabs flow into the same ocean?
yes they are ... but every branch has a different taste.
why is Imam Abu Hanifa seen as a bad person?
no muslim would say "Abu Hanifah is bad" ...but rather few muslims (basically those who belong to the Salafi movement ) would say "some of Abu Hanifah view are flawed" ....
that could be easily understood ,if we realize how the salafi school is greatly influenced of Imam Ibn Hanbal and those alike Ibn Taymiyyah etc......
Abu Hanifa gave much weight to the opinion ,while Ibn Hanbal gave much weight to the traditions.... and that is where the disagreement , the conflict.