...So,

why are we killing this guy?

That guy's a drug dealer, and the client that requested the kill is also a drug dealer.

...Why don't you sit over here?

Does he want to eliminate the competition and monopolize the market? Seems like spending 20 million would put him in the red...

In smuggling, you can earn tens of millions just by wiping out even a single competing organization.

...but this client said

"I want to kill him for business."...

It's probably a matter of pride.

The client's main job is at a pharmaceutical company.

He was bragging about how he tried making illegal drugs out of curiosity and that they sold like hotcakes, but...

he seemed like a weak-willed person.

He was scared and implied he pissed off some bad guy and got his friends killed or something.

I'm sure he intends to wash his hands of the matter once he pays the 20 million.

Even if he can manufacture medicine, he doesn't have any business sense in this world.

We're risking our lives for someone so worthless...?