why is the senate bad?

It was designed for a very different model of government than the one we now have, existing in a very different world than the one we now live in.

Basically, the US was conceived originally as a federation of independent states. Each state would operate more-or-less like its own little country. Citizens of the states would elect their own governments, and then those governments would select delegates to represent them in the Senate. Imagine an arrangement very much like the EU, but with less centralized power.

This made sense in a world where travel was slow and arduous, where long-distance communication could take months, where economies were highly localized and landowners' interests were tied tightly to their geographic location, and (perhaps most importantly) where the only 'minorities' whose interests the framers were worried about protecting were white landowners in states with small white populations (i.e. the South - so much of the early history of the US is fundamentally about slavery.)

As technology improved, slavery was abolished, voting rights expanded, people started moving around more, economies diversified, and political philosophies evolved, the idea of 'states' as coherent entities with common interests became more and more obviously fictional. The federal government found itself repeatedly having to step in to protect people from their own state governments (something that continues even today).

For this and other reasons, the role of the federal government expanded and it began interfacing more and more directly with individuals and localities. It developed separate systems to help protect minority groups and interests.

The senate remains as a sort of relic of a time when the federal government thought of itself as governing states rather than governing people. It continues to give more weight to voters in rural states than in urban ones, but now it has a lot more power to affect the lives of individuals than the framers ever imagined it having.

(The framers were suspicious of democracy as applied to the federal government because the federal government was supposed to operate at a level far abstracted from the individual needs of voters. They quite supported democracy (for white male landowners) in the lower levels of government that directly affected voters' lives.)